On Thu, Apr 12, 2018 at 01:21:42PM +0200, Wolfram Sang wrote: > > > > +static unsigned long global_flags; > > > > Is the restriction on concurrent DMA RX streams global or per-device? > > ? Each device has only one DMA RX channel. Hey Simon, you upstreamed > this driver :) Or did I get the question wrong? As I understand things this patch implements a restriction on concurrent DMA RX streams for old SoCs, corresponding with a limitation in the hardware. As the implementation stands it is global - only one DMA RX stream may be in flight for the entire system. I am wondering if that is the right granularity for the restriction. Perhaps it could be per-SDHI device, allowing concurrent streams on different SDHI devices. I think what you have is safe. But perhaps it could be relaxed. I do not have any insights regarding the extent of the hardware restriction (that I can recall at this time). > > > + if (dir == DMA_FROM_DEVICE) > > > + clear_bit(SDHI_INTERNAL_DMAC_RX_IN_USE, &global_flags); > > > + > > > > Is clear_bit() expensive? If so it might be worth avoiding on SoCs that > > don't have the restriction covered by this patch. > > It's an atoimc bitop, so maybe has a memory barrier. Hmm, the above > version is better on the cache lines, though, if you don't have the > restriction. > > Will think about it and make sure both clear_bit() are in sync. Thanks, I don't feel strongly about this.
