Hi Arnd,
On 06/14/2011 09:33 PM, Arnd Bergmann wrote:
On Tuesday 14 June 2011, Michal Nazarewicz wrote:
On Tue, 14 Jun 2011 15:49:29 +0200, Arnd Bergmann<arnd@xxxxxxxx> wrote:
Please explain the exact requirements that lead you to defining multiple
contexts.
Some devices may have access only to some banks of memory. Some devices
may use different banks of memory for different purposes.
For all I know, that is something that is only true for a few very special
Samsung devices, and is completely unrelated of the need for contiguous
allocations, so this approach becomes pointless as soon as the next
generation of that chip grows an IOMMU, where we don't handle the special
bank attributes. Also, the way I understood the situation for the Samsung
SoC during the Budapest discussion, it's only a performance hack, not a
functional requirement, unless you count '1080p playback' as a functional
requirement.
1080p@30fps is indeed a functional requirement, as the IP has the
capability to achieve it. This IP itself can act as more than one AXI
master, and control more than one memory port(bank). So this is not a
*performance hack*
Also, if I recall, during the Budapest discussion (I was on irc), I
recall that this requirement can become the information available to the
actual allocator. Below is the summary point I could collect from summit
notes:
* May also need to specify more attributes (specific physical
memory region)
As per this point, the requirement (as above) must be attribute to the
allocator, which is CMA in this case.
Supporting contiguous allocation is a very useful goal and many people want
this, but supporting a crazy one-off hardware design with lots of generic
infrastructure is going a bit too far. If you can't be more specific than
'some devices may need this', I would suggest going forward without having
multiple regions:
* Remove the registration of specific addresses from the initial patch
set (but keep the patch).
* Add a heuristic plus command-line override to automatically come up
with a reasonable location+size for *one* CMA area in the system.
* Ship the patch to add support for multiple CMA areas with the BSP
for the boards that need it (if any).
* Wait for someone on a non-Samsung SoC to run into the same problem,
then have /them/ get the final patch in.
Even if you think you can convince enough people that having support
for distinct predefined regions is a good idea, I would recommend
splitting that out of the initial merge so we can have that discussion
separately from the other issues.
Arnd
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Regards,
Subash
SISO-SLG
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