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From: Francisco Jerez <currojerez@riseup.net>
To: Ayaz A Siddiqui <ayaz.siddiqui@intel.com>,
	intel-gfx@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915/gt: Initialize reserved and unspecified MOCS indices
Date: Thu, 04 Jun 2020 14:34:57 -0700	[thread overview]
Message-ID: <87eequpla6.fsf@riseup.net> (raw)
In-Reply-To: <20200604182658.878417-1-ayaz.siddiqui@intel.com>


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Ayaz A Siddiqui <ayaz.siddiqui@intel.com> writes:

> In order to avoid functional breakage of mis-programmed applications that
> have grown to depend on unused MOCS entries, we are programming
> those entries to be equal to fully cached ("L3 + LLC") entry as per the
> recommendation from architecture team.
>
> These reserved and unspecified entries should not be used as they may be
> changed to less performant variants with better coherency in the future
> if more entries are needed.
>

This change seems highly questionable to me...  If a future kernel
release introduces a new MOCS entry with more strict coherency
semantics, and an application starts relying on it, that application
won't work when run on an older kernel version with this patch is
applied.  IOW setting uninitialized entries to the most strict caching
setting available (UC) ensures forwards compatibility with future
userspace, which seems like a more important design principle than
giving full caching to broken userspace that accidentally makes use of
an undefined MOCS entry not part of the kernel ABI.

> Signed-off-by: Ayaz A Siddiqui <ayaz.siddiqui@intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_mocs.c | 93 ++++++++++++++++++++++++++--
>  1 file changed, 89 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_mocs.c b/drivers/gpu/drm/i915/gt/intel_mocs.c
> index 632e08a4592b..1089bd5fdba2 100644
> --- a/drivers/gpu/drm/i915/gt/intel_mocs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_mocs.c
> @@ -234,10 +234,6 @@ static const struct drm_i915_mocs_entry broxton_mocs_table[] = {
>  		   L3_1_UC)
>  
>  static const struct drm_i915_mocs_entry tgl_mocs_table[] = {
> -	/* Base - Error (Reserved for Non-Use) */
> -	MOCS_ENTRY(0, 0x0, 0x0),
> -	/* Base - Reserved */
> -	MOCS_ENTRY(1, 0x0, 0x0),
>  
>  	GEN11_MOCS_ENTRIES,
>  
> @@ -265,6 +261,95 @@ static const struct drm_i915_mocs_entry tgl_mocs_table[] = {
>  	MOCS_ENTRY(61,
>  		   LE_1_UC | LE_TC_1_LLC,
>  		   L3_3_WB),
> +
> +	/* NOTE:
> +	 * Reserved and unspecified MOCS indices have been set to (L3 + LCC).
> +	 * These reserved entry should never be used, they may be chanaged
> +	 * to low performant variants with better coherency in the future if
> +	 * more entries are needed.
> +	 */
> +
> +	/* Reserved index 0 and 1 */
> +	MOCS_ENTRY(0, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(1, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +
> +	/* Reserved index 16 and 17 */
> +	MOCS_ENTRY(16, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(17, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +
> +	/* Reserved index 24 and 25 */
> +	MOCS_ENTRY(24, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(25, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +
> +	/* Unspecified indices 26 to 47 */
> +	MOCS_ENTRY(26, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(27, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(28, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(29, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(30, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(31, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(32, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(33, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(34, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(35, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(36, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(37, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(38, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(39, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(40, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(41, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(42, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(43, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(44, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(45, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(46, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(47, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +
> +	/* Unspecified indices 52 to 59 */
> +	MOCS_ENTRY(52, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(53, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(54, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(55, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(56, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(57, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(58, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB),
> +	MOCS_ENTRY(59, LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_3_WB)
>  };
>  
>  static const struct drm_i915_mocs_entry icl_mocs_table[] = {
> -- 
> 2.26.2
>
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

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Intel-gfx@lists.freedesktop.org
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  parent reply	other threads:[~2020-06-04 21:35 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-04 18:26 [Intel-gfx] [PATCH v2] drm/i915/gt: Initialize reserved and unspecified MOCS indices Ayaz A Siddiqui
2020-06-04 19:08 ` [Intel-gfx] ✓ Fi.CI.BAT: success for " Patchwork
2020-06-04 21:34 ` Francisco Jerez [this message]
2020-06-08  9:47   ` [Intel-gfx] [PATCH v2] " Joonas Lahtinen
2020-06-09  1:39     ` Francisco Jerez
2020-06-04 23:10 ` [Intel-gfx] ✓ Fi.CI.IGT: success for " Patchwork

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