Re: [PATCH] dma: rcar-dmac: use list_add() on rcar_dmac_desc_put()

[Vinod Koul <vinod.koul@intel.com>]

On Wed, May 11, 2016 at 03:28:04AM +0000, Kuninori Morimoto wrote:
> 
> Hi Laurent, Vinod,
> 
> > > > From: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
> > > > 
> > > > 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