[PATCH v4 15/22] drm/tilcdc: Get rid of complex ping-pong mechanism

[Tomi Valkeinen]

On 24/02/16 22:31, Rob Clark wrote:
> On Wed, Feb 24, 2016 at 11:48 AM, Jyri Sarha <jsarha at ti.com> wrote:
>> From: Tomi Valkeinen <tomi.valkeinen at ti.com>
>>
>> Get rid of complex ping-pong mechanism and replace it with simpler
>> single buffer flipping code.
>>
>> The LCDC HW appears to be designed mainly static framebuffers in
>> mind. There are two modes of operation, either static single buffer,
>> or ping pong double buffering with two static buffers switching back
>> and forth. Luckily the framebuffer start address is fetched only in
>> the beginning of the frame and changing the address after that only
>> takes effect after the next vertical blank. The page flipping code can
>> simply write the address of the new framebuffer and the page is
>> flipped automatically after the next vertical blank. Using the ping
>> pong double buffering makes the flipping code way more complex and it
>> does not provide any benefit, so it is better to switch to single
>> buffer operation.
>>
>> There is still one problem in updating the framebuffer dma address on
>> the fly. There are two registers defining the framebuffer dma area and
>> things may break if the dma address is fetched in while the registers
>> are are being updated.
> 
> Just curious, but does this mean you need to avoid writing the reg's
> too near to vblank?  (I think there are other drivers which do not

Yes.

> have double buffered registers and must do this.. but it's a pain..)
> 
> The ping-pong mode is super-wonky, and would be nice to get rid of..
> but with this scheme I'm not quite sure what happens if a pageflip
> comes in too close to vblank..

It's the same problem with both single and double buffer (ping-pong)
modes. The HW is designed to have a static configuration of one or two
buffers, never changed at runtime. If you change the buffers at runtime,
and change them exactly at the bad time during vblank when the HW starts
using the start and end addresses, you end up with broken display.

In other words, using the double buffering mode only adds extra
complexity to the driver, without any benefit. In fact, with the
double-buffering mode I think it's impossible to make the system
reliable. The driver cannot reliably track which of those two buffers
are currently shown on the screen, so when you do a page flip, you could
flip the wrong buffer.

The next patch adds some safety margin for the "page-flip near vblank"
issue.

 Tomi

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