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Date:   Mon, 30 Dec 2019 15:33:49 +0800
From:   Xu Yilum <yilun.xu@intel.com>
To:     Moritz Fischer <mdf@kernel.org>
Cc:     linux-fpga@vger.kernel.org, linux-kernel@vger.kernel.org,
        hao.wu@intel.com
Subject: Re: [PATCH] fpga: dfl: support multiple opens on feature device node.
Message-ID: <20191230073349.GA30202@yilunxu-OptiPlex-7050>
References: <1574054441-1568-1-git-send-email-yilun.xu@intel.com>
 <20191210020654.GA7171@archbook>
 <20191210085435.GA17198@yilunxu-OptiPlex-7050>
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Hi Moritz:

Do you have more concern on this patch?

I've sent a patch v2 for this, fixed minor inline comments.

Thanks.

On Tue, Dec 10, 2019 at 04:54:36PM +0800, Xu Yilun wrote:
> On Mon, Dec 09, 2019 at 06:06:54PM -0800, Moritz Fischer wrote:
> > Hi Xu,
> > 
> > Sorry for the delay.
> > 
> > On Mon, Nov 18, 2019 at 01:20:41PM +0800, Xu Yilun wrote:
> > > Each DFL functional block, e.g. AFU (Accelerated Function Unit) and FME
> > > (FPGA Management Engine), could implement more than one function within
> > > its region, but current driver only allows one user application to access
> > > it by exclusive open on device node. So this is not convenient and
> > > flexible for userspace applications, as they have to combine lots of
> > > different functions into one single application.
> > I'm not entirely sure how I feel about that, wouldn't the right solution
> > be to create more devices? If this ends up being 100% passthrough
> > basically, VFIO would be a better choice?
> 
> The major issue we want to resolve is how to access the same accelerator by
> multiple applications. As you know, the accelerator is the dynamic region (AFU)
> defined by end user, so we don't know how many sub functions end user will
> implement in the one dynamic region (AFU) of FPGA. So driver can't create devices
> for logic blocks implemented inside dynamic region.
> 
> We don't want 100% passthrough, but only dynamic region (AFU), because for static
> region part, we can reuse everything, including hardware logics, kernel drivers and
> upper layer stack/lib/app/tools. Use VFIO PCI directly may not be helpful for our case,
> and make the device node to support multiple open seems the simplest solution.
> 
> > > 
> > > This patch removes the limitation here to allow multiple opens to each
> > > feature device node for AFU and FME from userspace applications. If user
> > > still needs exclusive access to these device node, O_EXCL flag must be
> > > issued together with open.
> > Wouldn't you wanna default to have exclusive access, and then allow
> > multiple opens by means of a management interface, maybe?
> 
> I see some examples here which is using O_EXCL flag to indicate exclusive open,
> like nvram misc driver. I feel this may be a regular method to follow, I believe
> if we can save one userspace interface with this, that will be simpler for users
> too.
> 
> > > 
> > > Signed-off-by: Wu Hao <hao.wu@intel.com>
> > > Signed-off-by: Xu Yilun <yilun.xu@intel.com>
> > > ---
> > >  drivers/fpga/dfl-afu-main.c | 26 +++++++++++++++-----------
> > >  drivers/fpga/dfl-fme-main.c | 19 ++++++++++++-------
> > >  drivers/fpga/dfl.c          | 15 +++++++++++++--
> > >  drivers/fpga/dfl.h          | 35 +++++++++++++++++++++++++++--------
> > >  4 files changed, 67 insertions(+), 28 deletions(-)
> > > 
> > > diff --git a/drivers/fpga/dfl-afu-main.c b/drivers/fpga/dfl-afu-main.c
> > > index e4a34dc..c6e0e07 100644
> > > --- a/drivers/fpga/dfl-afu-main.c
> > > +++ b/drivers/fpga/dfl-afu-main.c
> > > @@ -561,14 +561,16 @@ static int afu_open(struct inode *inode, struct file *filp)
> > >  	if (WARN_ON(!pdata))
> > >  		return -ENODEV;
> > >  
> > > -	ret = dfl_feature_dev_use_begin(pdata);
> > > -	if (ret)
> > > -		return ret;
> > > -
> > > -	dev_dbg(&fdev->dev, "Device File Open\n");
> > > -	filp->private_data = fdev;
> > > +	mutex_lock(&pdata->lock);
> > > +	ret = dfl_feature_dev_use_begin(pdata, filp->f_flags & O_EXCL);
> > > +	if (!ret) {
> > > +		dev_dbg(&fdev->dev, "Device File Opened %d Times\n",
> > > +			dfl_feature_dev_use_count(pdata));
> > Could those be trace events instead?
> 
> I investigated a little to see how we use trace to get this info.
> 1. config PROFILE_ANNOTATED_BRANCHES in kernel, and add likely()/unlikely()
>    for ret judgement. This adds some overhead to system.
> 2. config CONFIG_PROFILE_ALL_BRANCHES, no code change here. This adds
>    great overhead to system.
> 3. write a dedicated trace event for this count. I think this is a little
>    too heavy, we need to add extra file for it, but this count is just
>    minor info to the driver.
> 
> What's your suggestion for this? Could we just leave the print here for
> simplicity.
> 
> 
> > > +		filp->private_data = fdev;
> > > +	}
> > > +	mutex_unlock(&pdata->lock);
> > >  
> > > -	return 0;
> > > +	return ret;
> > >  }
> > >  
> > >  static int afu_release(struct inode *inode, struct file *filp)
> > > @@ -581,12 +583,14 @@ static int afu_release(struct inode *inode, struct file *filp)
> > >  	pdata = dev_get_platdata(&pdev->dev);
> > >  
> > >  	mutex_lock(&pdata->lock);
> > > -	__port_reset(pdev);
> > > -	afu_dma_region_destroy(pdata);
> > > -	mutex_unlock(&pdata->lock);
> > > -
> > >  	dfl_feature_dev_use_end(pdata);
> > >  
> > > +	if (!dfl_feature_dev_use_count(pdata)) {
> > > +		__port_reset(pdev);
> > > +		afu_dma_region_destroy(pdata);
> > > +	}
> > > +	mutex_unlock(&pdata->lock);
> > > +
> > >  	return 0;
> > >  }
> > >  
> > > diff --git a/drivers/fpga/dfl-fme-main.c b/drivers/fpga/dfl-fme-main.c
> > > index 7c930e6..fda8623 100644
> > > --- a/drivers/fpga/dfl-fme-main.c
> > > +++ b/drivers/fpga/dfl-fme-main.c
> > > @@ -600,14 +600,16 @@ static int fme_open(struct inode *inode, struct file *filp)
> > >  	if (WARN_ON(!pdata))
> > >  		return -ENODEV;
> > >  
> > > -	ret = dfl_feature_dev_use_begin(pdata);
> > > -	if (ret)
> > > -		return ret;
> > > -
> > > -	dev_dbg(&fdev->dev, "Device File Open\n");
> > > -	filp->private_data = pdata;
> > > +	mutex_lock(&pdata->lock);
> > > +	ret = dfl_feature_dev_use_begin(pdata, filp->f_flags & O_EXCL);
> > > +	if (!ret) {
> > > +		dev_dbg(&fdev->dev, "Device File Opened %d Times\n",
> > > +			dfl_feature_dev_use_count(pdata));
> > > +		filp->private_data = pdata;
> > > +	}
> > > +	mutex_unlock(&pdata->lock);
> > >  
> > > -	return 0;
> > > +	return ret;
> > >  }
> > >  
> > >  static int fme_release(struct inode *inode, struct file *filp)
> > > @@ -616,7 +618,10 @@ static int fme_release(struct inode *inode, struct file *filp)
> > >  	struct platform_device *pdev = pdata->dev;
> > >  
> > >  	dev_dbg(&pdev->dev, "Device File Release\n");
> > > +
> > > +	mutex_lock(&pdata->lock);
> > >  	dfl_feature_dev_use_end(pdata);
> > > +	mutex_unlock(&pdata->lock);
> > >  
> > >  	return 0;
> > >  }
> > > diff --git a/drivers/fpga/dfl.c b/drivers/fpga/dfl.c
> > > index 96a2b82..9909948 100644
> > > --- a/drivers/fpga/dfl.c
> > > +++ b/drivers/fpga/dfl.c
> > > @@ -1079,6 +1079,7 @@ static int __init dfl_fpga_init(void)
> > >   */
> > >  int dfl_fpga_cdev_release_port(struct dfl_fpga_cdev *cdev, int port_id)
> > >  {
> > > +	struct dfl_feature_platform_data *pdata;
> > >  	struct platform_device *port_pdev;
> > >  	int ret = -ENODEV;
> > >  
> > > @@ -1093,7 +1094,11 @@ int dfl_fpga_cdev_release_port(struct dfl_fpga_cdev *cdev, int port_id)
> > >  		goto put_dev_exit;
> > >  	}
> > >  
> > > -	ret = dfl_feature_dev_use_begin(dev_get_platdata(&port_pdev->dev));
> > > +	pdata = dev_get_platdata(&port_pdev->dev);
> > > +
> > > +	mutex_lock(&pdata->lock);
> > > +	ret = dfl_feature_dev_use_begin(pdata, true);
> > > +	mutex_unlock(&pdata->lock);
> > >  	if (ret)
> > >  		goto put_dev_exit;
> > >  
> > > @@ -1120,6 +1125,7 @@ EXPORT_SYMBOL_GPL(dfl_fpga_cdev_release_port);
> > >   */
> > >  int dfl_fpga_cdev_assign_port(struct dfl_fpga_cdev *cdev, int port_id)
> > >  {
> > > +	struct dfl_feature_platform_data *pdata;
> > >  	struct platform_device *port_pdev;
> > >  	int ret = -ENODEV;
> > >  
> > > @@ -1138,7 +1144,12 @@ int dfl_fpga_cdev_assign_port(struct dfl_fpga_cdev *cdev, int port_id)
> > >  	if (ret)
> > >  		goto put_dev_exit;
> > >  
> > > -	dfl_feature_dev_use_end(dev_get_platdata(&port_pdev->dev));
> > > +	pdata = dev_get_platdata(&port_pdev->dev);
> > > +
> > > +	mutex_lock(&pdata->lock);
> > > +	dfl_feature_dev_use_end(pdata);
> > > +	mutex_unlock(&pdata->lock);
> > > +
> > >  	cdev->released_port_num--;
> > >  put_dev_exit:
> > >  	put_device(&port_pdev->dev);
> > > diff --git a/drivers/fpga/dfl.h b/drivers/fpga/dfl.h
> > > index 9f0e656..4a9a33c 100644
> > > --- a/drivers/fpga/dfl.h
> > > +++ b/drivers/fpga/dfl.h
> > > @@ -205,8 +205,6 @@ struct dfl_feature {
> > >  	const struct dfl_feature_ops *ops;
> > >  };
> > >  
> > > -#define DEV_STATUS_IN_USE	0
> > > -
> > >  #define FEATURE_DEV_ID_UNUSED	(-1)
> > >  
> > >  /**
> > > @@ -219,8 +217,9 @@ struct dfl_feature {
> > >   * @dfl_cdev: ptr to container device.
> > >   * @id: id used for this feature device.
> > >   * @disable_count: count for port disable.
> > > + * @excl_open: set on feature device exclusive open.
> > > + * @open_count: count for feature device open.
> > >   * @num: number for sub features.
> > > - * @dev_status: dev status (e.g. DEV_STATUS_IN_USE).
> > >   * @private: ptr to feature dev private data.
> > >   * @features: sub features of this feature dev.
> > >   */
> > > @@ -232,26 +231,46 @@ struct dfl_feature_platform_data {
> > >  	struct dfl_fpga_cdev *dfl_cdev;
> > >  	int id;
> > >  	unsigned int disable_count;
> > > -	unsigned long dev_status;
> > > +	bool excl_open;
> > > +	int open_count;
> > >  	void *private;
> > >  	int num;
> > >  	struct dfl_feature features[0];
> > >  };
> > >  
> > >  static inline
> > 
> > > -int dfl_feature_dev_use_begin(struct dfl_feature_platform_data *pdata)
> > > +int dfl_feature_dev_use_begin(struct dfl_feature_platform_data *pdata,
> > > +			      bool excl)
> > >  {
> > > -	/* Test and set IN_USE flags to ensure file is exclusively used */
> > > -	if (test_and_set_bit_lock(DEV_STATUS_IN_USE, &pdata->dev_status))
> > > +	if (pdata->excl_open)
> > >  		return -EBUSY;
> > >  
> > > +	if (excl) {
> > > +		if (pdata->open_count)
> > > +			return -EBUSY;
> > > +
> > > +		pdata->excl_open = true;
> > > +	}
> > > +	pdata->open_count++;
> > > +
> > >  	return 0;
> > >  }
> > >  
> > >  static inline
> > >  void dfl_feature_dev_use_end(struct dfl_feature_platform_data *pdata)
> > >  {
> > > -	clear_bit_unlock(DEV_STATUS_IN_USE, &pdata->dev_status);
> > > +	pdata->excl_open = false;
> > > +
> > > +	if (WARN_ON(pdata->open_count <= 0))
> > > +		return;
> > > +
> > > +	pdata->open_count--;
> > > +}
> > > +
> > > +static inline
> > Feel free to drop the inline here. Your compiler will figure that out by
> > iteself.
> 
> Yes I'll change it.
> 
> > > +int dfl_feature_dev_use_count(struct dfl_feature_platform_data *pdata)
> > > +{
> > > +	return pdata->open_count;
> > >  }
> > >  
> > >  static inline
> > > -- 
> > > 2.7.4
> > > 
> > 
> > Again, sorry for the delay,
> > 
> > Moritz
> 
> Thanks for take your time out :)
> 
> Yilun