On Wed, May 11, 2016 at 03:28:04AM +0000, Kuninori Morimoto wrote: > > Hi Laurent, Vinod, > > > > > From: Kuninori Morimoto <kuninori.morimoto.gx@xxxxxxxxxxx> > > > > > > > > Current rcar_dmac_desc_put() is using list_add_tail() in order to > > > > push used descriptor to list of free descriptors, and next DMA transfer > > > > try to reuse it from this list. But because it is using *_tail(), > > > > this reuse effect can't be obtained without using all of them. > > > > For a longer-term solution, we should allocate hardware descriptors > > > > using GFP_KERNEL instead of GFP_NOWAIT, but it is difficult today. > > > > This patch uses list_add() instead of list_add_tail() for short-term > > > > solution. > > > > > > So how does reuse case help by not moving the descriptor to tail. > > > > > > Also you are not reusing descriptor, you are reusing a descriptor memory, > > > these are two different things. > > > > > > Lastly how does this help? Something doesn't seem right > > > > For each descriptor, in addition to the memory used by the descriptors > > structure itself, the driver allocates a list of chunks as well as a buffer > > for hardware descriptors. Descriptors themselves are preallocated, and > > allocation of the chunks and buffer is performed the first time the descriptor > > is used. The memory isn't freed when the transfer is completed, as the chunks > > and buffer will be needed again when the descriptor is reused internally, so > > the driver keeps the memory around. > > > > If only a few descriptors are used concurrently, the current list_add_tail() > > implementation will result in all preallocated descriptors being used before > > going back to the first one, and will thus allocate chunks and a buffer for > > all preallocated descriptors. Using list_add() will put the complete > > descriptor at the head of the list of available descriptors, so the next > > transfer will be more likely to reuse a descriptor that already has associated > > memory instead of one that has never been used before. > > Laurent, thank you for your help > Vinod, does above clear for you ? Yes makese sense now. But please add these details in the changelog. This helps people know why a line was modified down the line -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
