linux-kernel - Re: [PATCH] drm/dp: Actually read Adjust Request Post Cursor2 register

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Date:   Thu, 9 Dec 2021 17:20:45 -0500
From:   Harry Wentland <harry.wentland@....com>
To:     Kees Cook <keescook@...omium.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     David Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-hardening@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH] drm/dp: Actually read Adjust Request Post Cursor2
 register



On 2021-12-09 01:23, Kees Cook wrote:
> On Wed, Dec 08, 2021 at 01:19:28PM +0200, Jani Nikula wrote:
>> On Fri, 03 Dec 2021, Kees Cook <keescook@...omium.org> wrote:
>>> The link_status array was not large enough to read the Adjust Request
>>> Post Cursor2 register. Adjust the size to include it. Found with a
>>> -Warray-bounds build:
>>>
>>> drivers/gpu/drm/drm_dp_helper.c: In function 'drm_dp_get_adjust_request_post_cursor':
>>> drivers/gpu/drm/drm_dp_helper.c:59:27: error: array subscript 10 is outside array bounds of 'const u8[6]' {aka 'const unsigned char[6]'} [-Werror=array-bounds]
>>>    59 |         return link_status[r - DP_LANE0_1_STATUS];
>>>       |                ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
>>> drivers/gpu/drm/drm_dp_helper.c:147:51: note: while referencing 'link_status'
>>>   147 | u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE],
>>>       |                                          ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> Fixes: 79465e0ffeb9 ("drm/dp: Add helper to get post-cursor adjustments")
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>
>> Using DP_ADJUST_REQUEST_POST_CURSOR2 has been deprecated since DP 1.3
>> published in 2014, and Tegra is the only user of
>> drm_dp_get_adjust_request_post_cursor().
> 
> I see POST_CURSOR2 is used here too:
> 
> drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
> 

Looks like we read and parse that in the admgpu driver without
using drm_dp_get_adjust_request_post_cursor.

I don't have a strong feeling but I liked your original
patch a bit better. I'm not sure what it means when part
of a spec is deprecated. Once a spec is written display
vendors might implement it. We should make sure that
displays like that are always handled in a sane manner.

Harry

> Here's a version of that for tegra (untested):
> 
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 23f9073bc473..c9528aa62c9c 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -144,16 +144,6 @@ u8 drm_dp_get_adjust_tx_ffe_preset(const u8 link_status[DP_LINK_STATUS_SIZE],
>  }
>  EXPORT_SYMBOL(drm_dp_get_adjust_tx_ffe_preset);
>  
> -u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE],
> -					 unsigned int lane)
> -{
> -	unsigned int offset = DP_ADJUST_REQUEST_POST_CURSOR2;
> -	u8 value = dp_link_status(link_status, offset);
> -
> -	return (value >> (lane << 1)) & 0x3;
> -}
> -EXPORT_SYMBOL(drm_dp_get_adjust_request_post_cursor);
> -
>  static int __8b10b_clock_recovery_delay_us(const struct drm_dp_aux *aux, u8 rd_interval)
>  {
>  	if (rd_interval > 4)
> diff --git a/drivers/gpu/drm/tegra/dp.c b/drivers/gpu/drm/tegra/dp.c
> index 70dfb7d1dec5..f5535eb04c6b 100644
> --- a/drivers/gpu/drm/tegra/dp.c
> +++ b/drivers/gpu/drm/tegra/dp.c
> @@ -549,6 +549,15 @@ static void drm_dp_link_get_adjustments(struct drm_dp_link *link,
>  {
>  	struct drm_dp_link_train_set *adjust = &link->train.adjust;
>  	unsigned int i;
> +	u8 post_cursor;
> +	int err;
> +
> +	err = drm_dp_dpcd_read(link->aux, DP_ADJUST_REQUEST_POST_CURSOR2,
> +			       &post_cursor, sizeof(post_cursor));
> +	if (err < 0) {
> +		DRM_ERROR("failed to read post_cursor2: %d\n", err);
> +		post_cursor = 0;
> +	}
>  
>  	for (i = 0; i < link->lanes; i++) {
>  		adjust->voltage_swing[i] =
> @@ -560,7 +569,7 @@ static void drm_dp_link_get_adjustments(struct drm_dp_link *link,
>  				DP_TRAIN_PRE_EMPHASIS_SHIFT;
>  
>  		adjust->post_cursor[i] =
> -			drm_dp_get_adjust_request_post_cursor(status, i);
> +			(post_cursor >> (i << 1)) & 0x3;
>  	}
>  }
>  
> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> index 30359e434c3f..28378db676c8 100644
> --- a/include/drm/drm_dp_helper.h
> +++ b/include/drm/drm_dp_helper.h
> @@ -1528,8 +1528,6 @@ u8 drm_dp_get_adjust_request_pre_emphasis(const u8 link_status[DP_LINK_STATUS_SI
>  					  int lane);
>  u8 drm_dp_get_adjust_tx_ffe_preset(const u8 link_status[DP_LINK_STATUS_SIZE],
>  				   int lane);
> -u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE],
> -					 unsigned int lane);
>  
>  #define DP_BRANCH_OUI_HEADER_SIZE	0xc
>  #define DP_RECEIVER_CAP_SIZE		0xf
> 
> 
> Is that the right way to go?
>