linux-kernel - Re: [PATCH V3 4/5] drm/vkms: Compute CRC without change input data

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Message-ID: <20190712031449.3pmeimkcde2hrxxh@smtp.gmail.com>
Date:   Fri, 12 Jul 2019 00:14:49 -0300
From:   Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
To:     Brian Starkey <brian.starkey@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        Simon Ser <contact@...rsion.fr>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 4/5] drm/vkms: Compute CRC without change input data

On 07/11, Daniel Vetter wrote:
> On Tue, Jun 25, 2019 at 10:38:31PM -0300, Rodrigo Siqueira wrote:
> > The compute_crc() function is responsible for calculating the
> > framebuffer CRC value; due to the XRGB format, this function has to
> > ignore the alpha channel during the CRC computation. Therefore,
> > compute_crc() set zero to the alpha channel directly in the input
> > framebuffer, which is not a problem since this function receives a copy
> > of the original buffer. However, if we want to use this function in a
> > context without a buffer copy, it will change the initial value. This
> > patch makes compute_crc() calculate the CRC value without modifying the
> > input framebuffer.
> 
> Uh why? For writeback we're writing the output too, so we can write
> whatever we want to into the alpha channel. For writeback we should never
> accept a pixel format where alpha actually matters, that doesn't make
> sense. You can't see through a real screen either, they are all opaque :-)
> -Daniel

Hmmm,

I see your point and I agree, but even though we can write whatever we
want in the output, don’t you think that is weird to change the
framebuffer value in the compute_crc() function?

Thanks
 
> > 
> > Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> > ---
> >  drivers/gpu/drm/vkms/vkms_composer.c | 31 +++++++++++++++++-----------
> >  1 file changed, 19 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> > index 51a270514219..8126aa0f968f 100644
> > --- a/drivers/gpu/drm/vkms/vkms_composer.c
> > +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> > @@ -6,33 +6,40 @@
> >  #include <drm/drm_atomic_helper.h>
> >  #include <drm/drm_gem_framebuffer_helper.h>
> >  
> > +static u32 get_pixel_from_buffer(int x, int y, const u8 *buffer,
> > +				 const struct vkms_composer *composer)
> > +{
> > +	int src_offset = composer->offset + (y * composer->pitch)
> > +					  + (x * composer->cpp);
> > +
> > +	return *(u32 *)&buffer[src_offset];
> > +}
> > +
> >  /**
> >   * compute_crc - Compute CRC value on output frame
> >   *
> > - * @vaddr_out: address to final framebuffer
> > + * @vaddr: address to final framebuffer
> >   * @composer: framebuffer's metadata
> >   *
> >   * returns CRC value computed using crc32 on the visible portion of
> >   * the final framebuffer at vaddr_out
> >   */
> > -static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
> > +static uint32_t compute_crc(const u8 *vaddr,
> > +			    const struct vkms_composer *composer)
> >  {
> > -	int i, j, src_offset;
> > +	int x, y;
> >  	int x_src = composer->src.x1 >> 16;
> >  	int y_src = composer->src.y1 >> 16;
> >  	int h_src = drm_rect_height(&composer->src) >> 16;
> >  	int w_src = drm_rect_width(&composer->src) >> 16;
> > -	u32 crc = 0;
> > +	u32 crc = 0, pixel = 0;
> >  
> > -	for (i = y_src; i < y_src + h_src; ++i) {
> > -		for (j = x_src; j < x_src + w_src; ++j) {
> > -			src_offset = composer->offset
> > -				     + (i * composer->pitch)
> > -				     + (j * composer->cpp);
> > +	for (y = y_src; y < y_src + h_src; ++y) {
> > +		for (x = x_src; x < x_src + w_src; ++x) {
> >  			/* XRGB format ignores Alpha channel */
> > -			memset(vaddr_out + src_offset + 24, 0,  8);
> > -			crc = crc32_le(crc, vaddr_out + src_offset,
> > -				       sizeof(u32));
> > +			pixel = get_pixel_from_buffer(x, y, vaddr, composer);
> > +			bitmap_clear((void *)&pixel, 0, 8);
> > +			crc = crc32_le(crc, (void *)&pixel, sizeof(u32));
> >  		}
> >  	}
> >  
> > -- 
> > 2.21.0
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Rodrigo Siqueira
https://siqueira.tech

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