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

[Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>]

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On 07/16, Daniel Vetter wrote:
> On Fri, Jul 12, 2019 at 12:14:49AM -0300, Rodrigo Siqueira wrote:
> > 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?
> 
> Not sure what you mean here ... ? From a quick look the memset only sets
> our temporary buffer, so we're not changing the input framebuffer here.
> And we have to somehow get rid of the X bits, since there's no alpha
> value. For CRC computation, all we need is some value which is the same
> for every frame (so that the CRC stays constant for the same visible
> output). For writeback we could write whatever we want (which includes
> whatever is there already). But there's no guarantee and definitely no
> expectation that the X bits survive. Writing 0 is imo the most reasonable
> thing to do. I'm not even sure whether modern gpus can still do channel
> masking (i.e. only write out specific channels, instead of the entire
> color). That was a "feature" of bitop blitters of the 80s/90s :-)
> -Daniel

Ahhh, now I can see that my mindset was in the 90s :)

Thanks
 
> > 
> > Thanks
> >  
> > > > 
> > > > Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.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
> 
> 
> 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Rodrigo Siqueira
https://siqueira.tech

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