Re: [PATCH] drm/sun4i: dsi: Prevent underflow when computing packet sizes

["Jernej Škrabec" <jernej.skrabec@gmail.com>]

Dne petek, 12. avgust 2022 ob 05:16:23 CEST je Samuel Holland napisal(a):
> Currently, the packet overhead is subtracted using unsigned arithmetic.
> With a short sync pulse, this could underflow and wrap around to near
> the maximal u16 value. Fix this by using signed subtraction. The call to
> max() will correctly handle any negative numbers that are produced.
> 
> Apply the same fix to the other timings, even though those subtractions
> are less likely to underflow.
> 
> Fixes: 133add5b5ad4 ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller
> support") Signed-off-by: Samuel Holland <samuel@sholland.org>
> ---
> 
>  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c index b4dfa166eccd..34234a144e87
> 100644
> --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> @@ -522,77 +522,77 @@ static void sun6i_dsi_setup_format(struct sun6i_dsi
> *dsi, SUN6I_DSI_PIXEL_PF1_CRC_INIT_LINE0(0xffff) |
>  		     SUN6I_DSI_PIXEL_PF1_CRC_INIT_LINEN(0xffff));
> 
>  	regmap_write(dsi->regs, SUN6I_DSI_PIXEL_CTL0_REG,
>  		     SUN6I_DSI_PIXEL_CTL0_PD_PLUG_DISABLE |
>  		     SUN6I_DSI_PIXEL_CTL0_FORMAT(fmt));
>  }
> 
>  static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
>  				    struct drm_display_mode 
*mode)
>  {
>  	struct mipi_dsi_device *device = dsi->device;
> -	unsigned int Bpp = mipi_dsi_pixel_format_to_bpp(device->format) / 
8;
> +	int Bpp = mipi_dsi_pixel_format_to_bpp(device->format) / 8;

Nit: mipi_dsi_pixel_format_to_bpp() can return -EINVAL in case of unsupported 
format. Would it make sense to check it?

In any case:
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>

Best regards,
Jernej

>  	u16 hbp = 0, hfp = 0, hsa = 0, hblk = 0, vblk = 0;
>  	u32 basic_ctl = 0;
>  	size_t bytes;
>  	u8 *buffer;
> 
>  	/* Do all timing calculations up front to allocate buffer space */
> 
>  	if (device->mode_flags & MIPI_DSI_MODE_VIDEO_BURST) {
>  		hblk = mode->hdisplay * Bpp;
>  		basic_ctl = SUN6I_DSI_BASIC_CTL_VIDEO_BURST |
>  			    SUN6I_DSI_BASIC_CTL_HSA_HSE_DIS |
>  			    SUN6I_DSI_BASIC_CTL_HBP_DIS;
> 
>  		if (device->lanes == 4)
>  			basic_ctl |= SUN6I_DSI_BASIC_CTL_TRAIL_FILL 
|
>  				     
SUN6I_DSI_BASIC_CTL_TRAIL_INV(0xc);
>  	} else {
>  		/*
>  		 * A sync period is composed of a blanking packet (4
>  		 * bytes + payload + 2 bytes) and a sync event packet
>  		 * (4 bytes). Its minimal size is therefore 10 bytes
>  		 */
>  #define HSA_PACKET_OVERHEAD	10
> -		hsa = max((unsigned int)HSA_PACKET_OVERHEAD,
> +		hsa = max(HSA_PACKET_OVERHEAD,
>  			  (mode->hsync_end - mode->hsync_start) * 
Bpp - HSA_PACKET_OVERHEAD);
> 
>  		/*
>  		 * The backporch is set using a blanking packet (4
>  		 * bytes + payload + 2 bytes). Its minimal size is
>  		 * therefore 6 bytes
>  		 */
>  #define HBP_PACKET_OVERHEAD	6
> -		hbp = max((unsigned int)HBP_PACKET_OVERHEAD,
> +		hbp = max(HBP_PACKET_OVERHEAD,
>  			  (mode->htotal - mode->hsync_end) * Bpp - 
HBP_PACKET_OVERHEAD);
> 
>  		/*
>  		 * The frontporch is set using a sync event (4 bytes)
>  		 * and two blanking packets (each one is 4 bytes +
>  		 * payload + 2 bytes). Its minimal size is therefore
>  		 * 16 bytes
>  		 */
>  #define HFP_PACKET_OVERHEAD	16
> -		hfp = max((unsigned int)HFP_PACKET_OVERHEAD,
> +		hfp = max(HFP_PACKET_OVERHEAD,
>  			  (mode->hsync_start - mode->hdisplay) * Bpp 
- HFP_PACKET_OVERHEAD);
> 
>  		/*
>  		 * The blanking is set using a sync event (4 bytes)
>  		 * and a blanking packet (4 bytes + payload + 2
>  		 * bytes). Its minimal size is therefore 10 bytes.
>  		 */
>  #define HBLK_PACKET_OVERHEAD	10
> -		hblk = max((unsigned int)HBLK_PACKET_OVERHEAD,
> +		hblk = max(HBLK_PACKET_OVERHEAD,
>  			   (mode->htotal - (mode->hsync_end - mode-
>hsync_start)) * Bpp -
>  			   HBLK_PACKET_OVERHEAD);
> 
>  		/*
>  		 * And I'm not entirely sure what vblk is about. The 
driver in
>  		 * Allwinner BSP is using a rather convoluted 
calculation
>  		 * there only for 4 lanes. However, using 0 (the !4 
lanes
>  		 * case) even with a 4 lanes screen seems to work...
>  		 */
>  		vblk = 0;
>  	}