Re: [PATCH 3/3] drm/i915: Allow optimized platform checks

Tvrtko Ursulin
2017-09-20T09:56:15Z

On 20/09/2017 10:39, Jani Nikula wrote:
> On Wed, 20 Sep 2017, Tvrtko Ursulin <tursulin@ursulin.net> wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> If we store the platform as a bitmask, and convert the
>> IS_PLATFORM macro to use it, we allow the compiler to
>> merge the IS_PLATFORM(a) || IS_PLATFORM(b) || ... checks
>> into a single conditional.
>>
>> Even with the added BUG_ON this saves almost 1k of text:
>>
>> text data bss dec hex filename
>> -1460254 60014 3656 1523924 1740d4 drivers/gpu/drm/i915/i915.ko
>> +1459260 60026 3656 1522942 173cfe drivers/gpu/drm/i915/i915.ko
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Cc: Jani Nikula <jani.nikula@intel.com>
>> ---
>> drivers/gpu/drm/i915/i915_drv.c | 6 ++++++
>> drivers/gpu/drm/i915/i915_drv.h | 3 ++-
>> 2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
>> index b1f96eb1be16..c3bd4b7cb19b 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.c
>> +++ b/drivers/gpu/drm/i915/i915_drv.c
>> @@ -869,6 +869,12 @@ static int i915_driver_init_early(struct drm_i915_private *dev_priv,
>> memcpy(device_info, match_info, sizeof(*device_info));
>> device_info->device_id = dev_priv->drm.pdev->device;
>>
>> + BUILD_BUG_ON(sizeof(device_info->platform_mask) * BITS_PER_BYTE <
>> + (INTEL_MAX_PLATFORMS - 1));
>> + BUG_ON(device_info->platform == 0 ||
>> + device_info->platform >= INTEL_MAX_PLATFORMS);
>> + device_info->platform_mask = BIT(device_info->platform - 1);
>
> Please just lose the -1, pretty please?
>
>> +
>> BUG_ON(device_info->gen > sizeof(device_info->gen_mask) * BITS_PER_BYTE);
>> device_info->gen_mask = BIT(device_info->gen - 1);
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
>> index 950aa109f8cb..81211f23326a 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.h
>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>> @@ -861,6 +861,7 @@ struct intel_device_info {
>> u8 ring_mask; /* Rings supported by the HW */
>>
>> enum intel_platform platform;
>> + u32 platform_mask;
>>
>> u32 display_mmio_offset;
>>
>> @@ -2984,7 +2985,7 @@ intel_info(const struct drm_i915_private *dev_priv)
>> #define IS_REVID(p, since, until) \
>> (INTEL_REVID(p) >= (since) && INTEL_REVID(p) <= (until))
>>
>> -#define IS_PLATFORM(dev_priv, p) ((dev_priv)->info.platform == (p))
>> +#define IS_PLATFORM(dev_priv, p) ((dev_priv)->info.platform_mask & BIT((p) - 1))
>
> What would the result be without platform_mask and just:
>
> #define IS_PLATFORM(dev_priv, p) (BIT((dev_priv)->info.platform) & BIT(p))

More code I'm afraid. But the problem of 32 platforms limit makes it
problematic for me. Because I checked, and going to u64 for the
platform_mask grows the code 100-200 bytes over the starting point. We'd
keep the single conditional advantage but I don't know, feels like not
worth it in that case. It could only last for a couple years before we
would need to go to u64.

Regards,

Tvrtko