[Intel-gfx] [PATCH v2] drm/i915: restore backlight precision when converting from opregion

[Aaron Lu]

On 05/04/2014 06:41 PM, Chris Wilson wrote:
> On Sun, May 04, 2014 at 03:31:01PM +0800, Aaron Lu wrote:
>> On 05/04/2014 03:22 PM, Chris Wilson wrote:
>>> Also this still has the same rounding error as before.
>>
>> I didn't get this, care to explain?
> 
> The calculation you use, truncates, rather than say round to nearest,
> would is the same discrepancy in your changelog.

This patch is meant to fix the problem that when user specify the max
backlight level in acpi_video interface, we will get the hardware max
level too in i915. With this patch, the hardware level we will get is:
level * freq / max = 255 * 937 / 255 = 937, which is correct.

For other values, the truncates is OK:
254 * 937 / 255 = 933
253 * 937 / 255 = 929
... ...