linux-kernel - Re: [PATCH] media: imx: Round line size to 4 bytes

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Date:   Sun, 17 Oct 2021 13:07:13 +0200
From:   Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Steve Longerbeam <slongerbeam@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...i.sm, phone-devel@...r.kernel.org
Subject: Re: [PATCH] media: imx: Round line size to 4 bytes

Hello,

On Thu, 14 Oct 2021 13:26:26 +0200
Philipp Zabel <p.zabel@...gutronix.de> wrote:

> Hi Dorota,
> 
> On Wed, 2021-10-13 at 11:26 +0200, Dorota Czaplejewicz wrote:
> > On Fri, 08 Oct 2021 14:19:41 +0200 Philipp Zabel <p.zabel@...gutronix.de> wrote:  
> [...]
> > > I wonder: if you use 4-byte aligned width and odd height, does the CSI
> > > write over the end of the buffer?  
> > 
> > I tested this case, and found a glitch which suggests the last 4 bytes are ignored:
> > 
> > https://source.puri.sm/Librem5/linux-next/uploads/cfb59e3832431aaa3a69549455502568/image.png  
> 
> Thank you for testing, so it appears that at least without FBUF_STRIDE
> the only requirement is that the whole image size must be a multiple of
> 8 bytes.
> 
> > That would be taken care of rounding up towards a number decided at runtime, like:
> > 
> > divisor = 8 >> (mbus->height % 2);  
> 
> Which would then cause the CSI to write past the end of the buffer?
> 
I'm not sure if you point out the mistake here (should be "4 <<"), or the fact that rounding is happening. If it's the latter, then it's of no concern: the values derived here are used to calculate buffer size.

I'm submitting a new series where this is fixed.

> I'd rather make sure that either the number of lines is even or the
> width is a multiple of 8 bytes.
> 
> > > > Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
> > > > ---
> > > > 
> > > > Hello,
> > > > 
> > > > my previous patch identified something that was not a problem,
> > > > so I'm sending a different one.
> > > > 
> > > > This has been tested on the Librem 5.
> > > > 
> > > > Cheers,
> > > > Dorota
> > > > 
> > > >  drivers/staging/media/imx/imx-media-utils.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/staging/media/imx/imx-media-utils.c b/drivers/staging/media/imx/imx-media-utils.c
> > > > index 5128915a5d6f..a303003929e3 100644
> > > > --- a/drivers/staging/media/imx/imx-media-utils.c
> > > > +++ b/drivers/staging/media/imx/imx-media-utils.c
> > > > @@ -545,13 +545,13 @@ int imx_media_mbus_fmt_to_pix_fmt(struct v4l2_pix_format *pix,
> > > >  	}
> > > >  
> > > >  	/* Round up width for minimum burst size */
> > > > -	width = round_up(mbus->width, 8);
> > > > +	width = round_up(mbus->width, 4);
> > > >  
> > > >  	/* Round up stride for IDMAC line start address alignment */
> > > >  	if (cc->planar)
> > > >  		stride = round_up(width, 16);
> > > >  	else
> > > > -		stride = round_up((width * cc->bpp) >> 3, 8);
> > > > +		stride = round_up((width * cc->bpp) >> 3, 4);    
> > > 
> > > Second, even if this works fine on the i.MX7/8M CSI, the alignment is
> > > still required for the i.MX5/6 IPU, for which this code and the comments
> > > were written. So we need a way to differentiate the two cases here.
> > > 
> > > regards
> > > Philipp  
> > 
> > How best to go about this? I can see in the file imx-media-capture.c
> > that there the video device lives in struct capture_priv.vdev.vfd.
> > Would that be the right place to query about the underlying hardware?
> > 
> > Then the following functions would gain a new "small_divisor" parameter:
> > - imx_media_mbus_fmt_to_pix_fmt (a GPL symbol)
> > - imx_media_mbus_fmt_to_ipu_image (a GPL symbol)
> > - __capture_try_fmt  
> 
> That sounds like it would work around the current code when it (at least
> part of imx_media_mbus_fmt_to_pix_fmt()) should be split between i.MX5/6
> and i.MX7/8 implementations. For example rounding up the stride is not
> useful on i.MX7/8, it just doesn't currently hurt because imx7-media-csi 
> is not using bytesperline to set up FBUF_STRIDE. And certainly the
> comments don't apply.
> 
> imx_media_mbus_fmt_to_ipu_image() is unused and should probably be
> dropped, same as imx_media_ipu_image_to_mbus_fmt().

Done in next series.
> 
> > Those would have to extract the device type from struct capture_priv:
> > - __capture_legacy_try_fmt
> > - capture_try_fmt_vid_cap
> > - capture_s_fmt_vid_cap
> > - capture_init_format  
> 
> Maybe imx_media_mbus_fmt_to_pix_fmt should be moved into imx-media-
> capture.c be passed struct capture_priv to avoid duplicating the device
> type check?
> 
I opted not to, in favor of passing the actual device type. It comes out to the same thing, except a simple value is passed around instead of a device.

> imx_media_capture_device_init() could gain a new parameter (or maybe
> replace legacy_api) to set the device type.

Thanks, this is what I was missing.

Regards,
Dorota
> 
> regards
> Philipp


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