Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 27 Jan 2020 12:40:52 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Jeffrey Hugo <jhugo@codeaurora.org>, arnd@arndb.de,
        smohanad@codeaurora.org, kvalo@codeaurora.org,
        bjorn.andersson@linaro.org, hemantk@codeaurora.org,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 05/16] bus: mhi: core: Add support for ringing
 channel/event ring doorbells
Message-ID: <20200127071052.GB4768@mani>
References: <20200123111836.7414-1-manivannan.sadhasivam@linaro.org>
 <20200123111836.7414-6-manivannan.sadhasivam@linaro.org>
 <beadf428-82db-c89f-22bc-983d7b907bb3@codeaurora.org>
 <20200125134631.GA3518689@kroah.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200125134631.GA3518689@kroah.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Jan 25, 2020 at 02:46:31PM +0100, Greg KH wrote:
> On Fri, Jan 24, 2020 at 03:51:12PM -0700, Jeffrey Hugo wrote:
> > > +struct mhi_event_ctxt {
> > > +	u32 reserved : 8;
> > > +	u32 intmodc : 8;
> > > +	u32 intmodt : 16;
> > > +	u32 ertype;
> > > +	u32 msivec;
> > > +
> > > +	u64 rbase __packed __aligned(4);
> > > +	u64 rlen __packed __aligned(4);
> > > +	u64 rp __packed __aligned(4);
> > > +	u64 wp __packed __aligned(4);
> > > +};
> > 
> > This is the struct that is shared with the device, correct?  Surely it needs
> > to be packed then?  Seems like you'd expect some padding between msivec and
> > rbase on a 64-bit system otherwise, which is probably not intended.
> > 
> > Also I strongly dislike bitfields in structures which are shared with
> > another system since the C specification doesn't define how they are
> > implemented, therefore you can run into issues where different compilers
> > decide to implement the actual backing memory differently.  I know its less
> > convinent, but I would prefer the use of bitmasks for these fields.
> 
> You have to use bitmasks in order for all endian cpus to work properly
> here, so that needs to be fixed.
> 

Okay.

> Oh, and if these values are in hardware, then the correct types also
> need to be used (i.e. __u32 and __u64).
> 

I thought the __* prefix types are only for sharing with userspace...
Could you please clarify why it is needed here?

Thanks,
Mani

> good catch!
> 
> greg k-h