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SubjectRe: [PATCH] drm/dp: Actually read Adjust Request Post Cursor2 register
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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@chromium.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@chromium.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?
>

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