Re: [PATCH] cleanup: Add 'struct dev' in the TTM layer to be passedin for DMA API calls.

From: Thomas Hellstrom
Date: Fri Apr 08 2011 - 11:30:24 EST

On 04/08/2011 05:12 PM, Konrad Rzeszutek Wilk wrote:
On Fri, Apr 08, 2011 at 04:57:14PM +0200, Thomas Hellstrom wrote:

Sorry for waiting so long to answer. Workload is quite heavy ATM.
Please see inline.
OK. Thank you for taking a look... some questions before you
depart on vacation.

1). Get in the patch that passed in 'struct dev' to the dma_alloc_coherent
for 2.6.39 so that PowerPC folks can use the it with radeon cards. My
understanding is that the work you plan on to isn't going in 2.6.39
but rather in 2.6.40 - and if get my stuff ready (the other phases)
we can work out the kinks together. This way also the 'struct dev'
is passed in the TTM layer.
I'm not happy with this solution. If something goes in, it should be
complete, otherwise future work need to worry about not breaking
something that's already broken. Also it adds things to TTM api's
that are not really necessary.

I'd like to see a solution that encapsulates all device-dependent
stuff (including the dma adresses) in the ttm backend, so the TTM
backend code is the only code that needs to worry about device
I am a bit confused here. The usual "ttm backend" refers to the
device specific hooks (so the radeon/nouveau/via driver), which
use this structure: ttm_backend_func

That is not what you are referring to right?

Yes, exactly.

dependent stuff. Core ttm should only need to worry about whether
pages can be transferrable to other devices, and whether pages can
be inserted into the page cache.
Ok. So the core ttm would need to know the 'struct dev' to figure
out what the criteria are for transferring the page (ie, it is
Ok for a 64-bit card to use a 32-bit card's pages, but not the other
way around)..

So the idea would be to have "ttm_backend::populate" decide whether the current pages are compatible with the device or not, and copy if that's the case.
Usually the pages are allocated by the backend itself and should be compatible, but the populate check would trigger if pages were transferred from another device. This case happens when the destination device has special requirements, and needs to be implemented in all backends when we start transfer TTMs between devices. Here we can use struct dev or something similar as a page compatibility identifier.

The other case is where the source device has special requirements, for example when the source device pages can't be inserted into the swap cache (This is the case you are referring to above). Core TTM does only need to know whether the pages are "normal pages" or not, and does not need to know about struct dev. Hence, the backend needs a query function, but not until we actually implement direct swap cache insertions.

So none of this stuff needs to be implemented now, and we can always hide struct dev in the backend.

This change should be pretty straightforward. We move the ttm::pages
array into the backend, and add ttm backend functions to allocate
pages and to free pages. The backend is then completely free to keep
track of page types and dma addresses completely hidden from core
ttm, and we don't need to shuffle those around. This opens up both
for completely device-private coherent memory and for "dummy device"
coherent memory.
The 'dummy device' is a bit of hack thought? Why not get rid
of that idea and just squirrel away the the 'struct dev' and let the
ttm::backend figure out how to allocate the pages?

Yes it's a hack. The advantage of a dummy device is that pages will be movable across backends that share the same dummy device. For example between a radeon and a nouveau driver on a Xen platform.

In the future, when TTM needs to move a ttm to another device, or
when it needs to insert pages into the page cache, pages that are
device specific will be copied and then freed. "Dummy device" pages
can be transferred to other devices, but not inserted into the page
OK. That would require some extra function in the ttm::backend to
say "dont_stick_this_in_page_cache".

Correct. We should use the ttm backend query function discussed above, and enumerate queries.


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