From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux.melwyn@gmail.com (melwyn lobo) Date: Wed, 14 Apr 2010 12:16:56 +0530 Subject: DMA Engine API performance issues In-Reply-To: References: Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Subject edited to allow posting on ARM kernel list On Wed, Apr 14, 2010 at 12:14 PM, melwyn lobo wrote: > Forwarding the query also to arm kernel mailiing list. > > Regards, > M. > > > ---------- Forwarded message ---------- > From: melwyn lobo > Date: Wed, Apr 14, 2010 at 12:12 PM > Subject: DMA Engine API performance issues > To: dan.j.williams at intel.com, linux-arm-kernel at lists.arm.linux.org.uk, > linux-kernel at vger.kernel.org > > > Hello Dan, > I have some doubts regarding DMA API usage on its clients for example > MMC, ALSA, USB etc. > > I am going to take example of the ALSA framework. Audio data transfer > is initiated in soc_pcm_trigger(). This is called in an atomic > context, > with spinlock held and irqs disabled. Here most drivers enable data > transfer from the MSP peripheral to the audio codec via tx_submit > implementation > of the DMA engine driver. This enqueues the transaction in an active > list which calls for using spinlocks with bottom half disabled. > It is in this function when spin_unlock_bh() is called the kernel > detects irq's disabled and generates a warning. > So the workaround here for ALSA drivers would be to use tasklet or > workqueues to defer calling this in an interruptible context, which > would > cause performance problems (the same function soc_pcm_trigger is > called for stoppping the transfer) in cases where the stream has to be > repeatedly > stopped and started. > > So the core issue is use of spin_unlock_bh in an atomic context. > Workaround for removing the warnings would be: > 1. Use spin_lock with irqsave and corresponding unlock function which > does not generate a warning in a similar situation. > ?But this could be futile in one case where the tasklet is scheduled > from ksoftirqd which could lead to corruption. > ?Also this means the interrupts would be disabled (on the local cpu) > till the function executed which is not something > ?desirable. > 2. Use local_irq_enable() before calling the DMA APIs and disable once > done. This is a crude solution and understandably undesirable and > dangerous. > > The DMA Engine framework assumes that the channel interrupt handling > is done in a tasklet (dma_run_dependencies), which I believe is the > reason for the issue. > > Regards, > M. >