Message-ID:
 <AS1PR04MB955915A33F184E141BCD69A2EAC12@AS1PR04MB9559.eurprd04.prod.outlook.com>
Date: Thu, 13 Jun 2024 05:32:52 +0000
From: Zhai He <zhai.he@....com>
To: Barry Song <baohua@...nel.org>
CC: Andrew Morton <akpm@...ux-foundation.org>, "sboyd@...nel.org"
	<sboyd@...nel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Zhipeng Wang
	<zhipeng.wang_1@....com>, Jindong Yue <jindong.yue@....com>, Christoph
 Hellwig <hch@....de>
Subject: RE: [EXT] Re: [PATCH v2] Supports to use the default CMA when the
 device-specified CMA memory is not enough.

> -----Original Message-----
> From: Barry Song <baohua@...nel.org>
> Sent: Thursday, June 13, 2024 11:28 AM
> To: Zhai He <zhai.he@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>; sboyd@...nel.org;
> linux-mm@...ck.org; linux-kernel@...r.kernel.org; Zhipeng Wang
> <zhipeng.wang_1@....com>; Jindong Yue <jindong.yue@....com>; Christoph
> Hellwig <hch@....de>
> Subject: Re: [EXT] Re: [PATCH v2] Supports to use the default CMA when the
> device-specified CMA memory is not enough.
> 
> Caution: This is an external email. Please take care when clicking links or
> opening attachments. When in doubt, report the message using the 'Report this
> email' button
> 
> 
> On Thu, Jun 13, 2024 at 2:34 PM Zhai He <zhai.he@....com> wrote:
> >
> > > -----Original Message-----
> > > From: Barry Song <baohua@...nel.org>
> > > Sent: Thursday, June 13, 2024 5:37 AM
> > > To: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: Zhai He <zhai.he@....com>; sboyd@...nel.org; linux-mm@...ck.org;
> > > linux-kernel@...r.kernel.org; stable@...r.kernel.org; Zhipeng Wang
> > > <zhipeng.wang_1@....com>; Jindong Yue <jindong.yue@....com>;
> > > Christoph Hellwig <hch@....de>
> > > Subject: [EXT] Re: [PATCH v2] Supports to use the default CMA when
> > > the device-specified CMA memory is not enough.
> > >
> > > Caution: This is an external email. Please take care when clicking
> > > links or opening attachments. When in doubt, report the message
> > > using the 'Report this email' button
> > >
> > >
> > > On Thu, Jun 13, 2024 at 6:47 AM Andrew Morton
> > > <akpm@...ux-foundation.org>
> > > wrote:
> > > >
> > > > On Wed, 12 Jun 2024 16:12:16 +0800 "zhai.he" <zhai.he@....com>
> wrote:
> > > >
> > > > > From: He Zhai <zhai.he@....com>
> > > >
> > > > (cc Barry & Christoph)
> > > >
> > > > What was your reason for adding cc:stable to the email headers?
> > > > Does this address some serious problem?  If so, please fully
> > > > describe that problem.
> > > >
> > > > > In the current code logic, if the device-specified CMA memory
> > > > > allocation fails, memory will not be allocated from the default CMA area.
> > > > > This patch will use the default cma region when the device's
> > > > > specified CMA is not enough.
> > > > >
> > > > > In addition, the log level of allocation failure is changed to debug.
> > > > > Because these logs will be printed when memory allocation from
> > > > > the device specified CMA fails, but if the allocation fails, it
> > > > > will be allocated from the default cma area. It can easily mislead
> developers'
> > > > > judgment.
> > >
> > > I am not convinced that this patch is correct. If device-specific
> > > CMA is too small, why not increase it in the device tree?
> > > Conversely, if the default CMA size is too large, why not reduce it
> > > via the cmdline?  CMA offers all kinds of flexible configuration options based
> on users’ needs.
> > >
> > > One significant benefit of device-specific CMA is that it helps
> > > decrease fragmentation in the common CMA pool. While many devices
> > > allocate memory from the same pool, they have different memory
> > > requirements in terms of sizes and alignments. Occasions of memory
> > > allocation and release can lead to situations where the CMA pool has
> > > enough free space, yet someone fails to obtain contiguous memory from it.
> > >
> > > This patch entirely negates the advantage we gain from device-specific CMA.
> > > My point is that instead of modifying the core code, please consider
> > > correcting your device tree or cmdline configurations.
> > >
> > Because we enabled secure heap to support widevine DRM, and secure
> > heap requires security configuration, its starting address cannot be
> > specified arbitrarily, which causes the default CMA to be reduced. So we
> reduced the CMA, but in order to avoid the impact of reducing the CMA, we
> used a multi-segment CMA and gave one segment to the VPU.
> >
> > However, under our memory configuration, the device-specific CMA is not
> enough to support the VPU decoding high-resolution code streams, so this patch
> is added so that the VPU can work properly.
> > Thanks.
> 
> I don’t quite understand what you are saying. Why can’t you increase VPU’s
> CMA size?
Thanks for your quick reply.
Because we added a secure heap to support Widevine DRM, this heap requires hardware protection, so its starting address cannot be specified arbitrarily. This causes the secure heap to occupy part of the default CMA, and the default CMA is therefore reduced, so in order to avoid default CMA Shrinking introduces other problems. We added a specific CMA area for the VPU. However, due to the large size of the secure heap and default CMA, There is no remaining memory available to increase specific CMA for VPU.

> It seems you mean that only in some corner cases do you need a large CMA, but
> most of the time, you don’t need it to be this big? So you have to "borrow"
> memory from the
> default CMA. but why not move that portion from the default CMA to your
> VPU’s CMA?
> 
This is a method, but because for VPU, the continuous memory size allocated by the driver is based on the video stream, we cannot determine the maximum size of memory required by the VPU. This makes it impossible for us to determine the size of the specific CMA assigned to the VPU. Thanks.
> Thanks
> Barry

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