From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joerg Roedel Subject: Re: [PATCH v2 1/4] dma-mapping: Add devm_ interface for dma_map_single() Date: Wed, 4 Jun 2014 01:39:07 +0200 Message-ID: <20140603233907.GB23880@8bytes.org> References: <1401606077-1739-1-git-send-email-eli.billauer@gmail.com> <1401606077-1739-2-git-send-email-eli.billauer@gmail.com> <538E3D04.9060808@samsung.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <538E3D04.9060808-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Shuah Khan Cc: devel-gWbeCf7V1WCQmaza687I9mD2FQJk+8+b@public.gmane.org, discuss-Wa75r+71Mmgdnm+yROfE0A@public.gmane.org, gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org, Eli Billauer , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, bhelgaas-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-pci-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org List-Id: iommu@lists.linux-foundation.org On Tue, Jun 03, 2014 at 03:24:20PM -0600, Shuah Khan wrote: > On 06/01/2014 01:01 AM, Eli Billauer wrote: > I see the value of this interface in unmap case, this type of wrapper > can release dma buffers, drivers neglected to release leaving dangling > buffers. > > However, driver writers should give it some thought before switching > from conventional dma_map_*() interfaces to these proposed managed > dma mapping interfaces. These new interfaces shouldn't be treated as > drop in replacements to dma_map_*() interfaces. > > The reasons are: > > 1. This interface adds an overhead in allocation memory for devres to > compared to other dma_map_* wrappers. Users need to be aware of that. > This would be okay in the cases where a driver allocates several > buffers at init time and uses them. However, several drivers allocate > during run-time and release as soon as it is no longer needed. This > overhead is going to be in the performance path. > > 2. It adds a failure case even when dma buffers are available. i.e if > if devres alloc fails, it will return failure even if dma map could > have succeeded. This is a new failure case for DMA-API. > > The reason dma_map_*() routines fail now is because there are no > buffers available. Drivers handle this error as a retry case. > > Drivers using dmam_map_single() will have to handle the failure > cases differently. > > Since the return values are different for dmam_map_*(), that is > plus that these interfaces can't be drop in replacements to the > dma_map_*() interfaces. > > 3. Similarly, it adds an overhead in releasing memory for devres to > compared to other dma_unmap_* wrappers. Users need to be aware of > that. This overhead is going to be in the performance path when > drivers unmap buffers during run-time. I fully agree with the points Shuah brought up here. I don't think it is a good idea to add this kind of resource management to runtime-allocated (and de-allocated) resources of device drivers. Also DMA handles are not something that could be garbage collected at driver unload time. They are a limited resource that may be used up at some point. And the whole point of a devm-API is that code can be simpler because we don't need to de-allocate everything on the error-path or at unload time, no? Besides that, we already have DMA-API debug to find drivers that do not release all their DMA buffers. Joerg