Re: [PATCH] drm/radeon: Disable writeback by default on ppc

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2013-12-05 at 11:29 +0900, Michel Dänzer wrote:
> On Don, 2013-12-05 at 12:39 +1100, Benjamin Herrenschmidt wrote:
> > On Wed, 2013-12-04 at 19:05 -0500, Alex Deucher wrote:
> > 
> > > > Setting CP_RB_CNTL.BUF_SWAP causes the CP to use the selected byte
> > > > swapping for just about everything accessed by the CP (rptr writeback,
> > > > indirect buffers, etc.).  Looks like the DMA ring supports and enables
> > > > rptr writeback as well (DMA_RB_CNTL.DMA_RPTR_WRITEBACK_SWAP_ENABLE) so
> > > > I think we can drop the swapping of the rptr writeback.
> > > >
> > > 
> > > Obvious patch attached.
> > 
> > This works all the way back to r300 ?
> 
> I don't think so, as I have writeback working without this patch on the
> RV350 in this PowerBook. So I think this function needs to be split,
> probably between R600 and older.

Or can we tell it to not swap (setup code) and continue doing
le32_to_cpu in the kernel ? Sounds better to me.

> Also, there's more code at least potentially affected by this, e.g. in: 
> 
>       * cik_compute_ring_get_rptr(), cik_compute_ring_get_wptr(),
>         cik_compute_ring_set_wptr(), cik_get_ih_wptr()
>       * si_get_ih_wptr()
>       * evergreen_get_ih_wptr()
>       * r600_get_ih_wptr()
>       * radeon_fence_write(), radeon_fence_read()
>       * radeon_ring_backup()

Yeah I'd say just don't swap, write LE to memory and let the kernel use
the right leXX_to_cpu.

HW swapping is evil :-)

Cheers,
Ben.


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