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From:   Andrea Merello <andrea.merello@gmail.com>
Date:   Fri, 28 Sep 2018 09:11:19 +0200
Message-ID: <CAN8YU5PWB0sRskcirCcYnY5vG5A_W6pcOMpgyOD3faWfMeu5Rg@mail.gmail.com>
Subject: Re: [PATCH v5 2/7] dmaengine: xilinx_dma: in axidma slave_sg and
 dma_cyclic mode align split descriptors
To:     Vinod <vkoul@kernel.org>
Cc:     dan.j.williams@intel.com, michal.simek@xilinx.com,
        appana.durga.rao@xilinx.com, dmaengine@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        devicetree <devicetree@vger.kernel.org>,
        Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
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On Tue, Sep 18, 2018 at 6:21 PM Vinod <vkoul@kernel.org> wrote:
>
> On 07-09-18, 08:24, Andrea Merello wrote:
> > Whenever a single or cyclic transaction is prepared, the driver
> > could eventually split it over several SG descriptors in order
> > to deal with the HW maximum transfer length.
> >
> > This could end up in DMA operations starting from a misaligned
> > address. This seems fatal for the HW if DRE (Data Realignment Engine)
> > is not enabled.
> >
> > This patch eventually adjusts the transfer size in order to make sure
> > all operations start from an aligned address.
> >
> > Cc: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
> > Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
> > Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
> > ---
> > Changes in v2:
> >         - don't introduce copy_mask field, rather rely on already-esistent
> >           copy_align field. Suggested by Radhey Shyam Pandey
> >         - reword title
> > Changes in v3:
> >       - fix bug introduced in v2: wrong copy size when DRE is enabled
> >       - use implementation suggested by Radhey Shyam Pandey
> > Changes in v4:
> >       - rework on the top of 1/6
> > Changes in v5:
> >       - fix typo in commit title
> >       - add hint about "DRE" meaning in commit message
> > ---
> >  drivers/dma/xilinx/xilinx_dma.c | 22 ++++++++++++++++++----
> >  1 file changed, 18 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
> > index a3aaa0e34cc7..aaa6de8a70e4 100644
> > --- a/drivers/dma/xilinx/xilinx_dma.c
> > +++ b/drivers/dma/xilinx/xilinx_dma.c
> > @@ -954,15 +954,28 @@ static int xilinx_dma_alloc_chan_resources(struct dma_chan *dchan)
> >
> >  /**
> >   * xilinx_dma_calc_copysize - Calculate the amount of data to copy
> > + * @chan: Driver specific DMA channel
> >   * @size: Total data that needs to be copied
> >   * @done: Amount of data that has been already copied
> >   *
> >   * Return: Amount of data that has to be copied
> >   */
> > -static int xilinx_dma_calc_copysize(int size, int done)
> > +static int xilinx_dma_calc_copysize(struct xilinx_dma_chan *chan,
> > +                                 int size, int done)
>
> align to preceeding line opening brace please

After applying, I'm seeing it already aligned as you requested; 4 tabs
+ 4 spaces so the 2nd line starts right under the "s" near the opened
brace..
Patch sent using git, so it should pass through without being ruined;
don't know why you see it misaligned :(

> >  {
> > -     return min_t(size_t, size - done,
> > +     size_t copy = min_t(size_t, size - done,
> >                    XILINX_DMA_MAX_TRANS_LEN);
>
> so we can do this way in patch 1:
>
>         size t copy;
>
>         copy =  min_t(size_t, size - done,
>                       XILINX_DMA_MAX_TRANS_LEN);
>
>         return copy;
>
> and then add these here, feels like we are redoing change introduced in
> patch 1..

OK, this sounds good :)

>
> > +     if ((copy + done < size) &&
> > +         chan->xdev->common.copy_align) {
> > +             /*
> > +              * If this is not the last descriptor, make sure
> > +              * the next one will be properly aligned
> > +              */
> > +             copy = rounddown(copy,
> > +                              (1 << chan->xdev->common.copy_align));
> > +     }
> > +     return copy;
> >  }
> >
> >  /**
> > @@ -1804,7 +1817,7 @@ static struct dma_async_tx_descriptor *xilinx_dma_prep_slave_sg(
> >                        * Calculate the maximum number of bytes to transfer,
> >                        * making sure it is less than the hw limit
> >                        */
> > -                     copy = xilinx_dma_calc_copysize(sg_dma_len(sg),
> > +                     copy = xilinx_dma_calc_copysize(chan, sg_dma_len(sg),
>
> why not keep chan in patch 1 and add only handling in patch 2, seems
> less churn to me..

Indeed this was something I was unsure about.. I ended up in feeling
better not to add introduce a function that takes an unused (yet)
argument, but I can change this of course :)

> --
> ~Vinod