On Thu, 18 Jul 2019 13:51:12 +0200 Petr Tesarik <ptesarik@xxxxxxx> wrote: > On Thu, 18 Jul 2019 13:36:33 +0200 > Heiko Carstens <heiko.carstens@xxxxxxxxxx> wrote: > > > On Thu, Jul 18, 2019 at 09:17:00AM +0200, Petr Tesarik wrote: > > > Hi all, > > > > > > while looking into DMA allocation, I noticed that > > > __dma_direct_optimal_gfp_mask() in kernel/dma/direct.c can probably be > > > improved. It uses GFP_DMA if dev->coherent_dma_mask is less than > > > DMA_BIT_MASK(ARCH_ZONE_DMA_BITS). There is no s390-specific definition > > > of ARCH_ZONE_DMA_BITS. The default is 24 bits, but the DMA zone on s390 > > > is 31 bits. CCW subchannel devices set sch->dev.coherent_dma_mask to > > > DMA_BIT_MASK(31), which is greater than DMA_BIT_MASK(24), so buffers > > > are allocated from the Normal zone first. > > > > > > Would it make sense to set ARCH_ZONE_BITS to 31 on s390, or did I miss > > > something? > > > > No, this seems to be broken. Halil, can you look into this and provide > > a patch? > > I wondered why the kernel works OK on my system, and it is in fact not > so bad. If the first allocation fails, the kernel adds GFP_DMA and > retries, so this is not fatal, but with a proper definition of > ARCH_ZONE_DMA_BITS it should be possible to get success in the first > attempt already, let's do it. > > Petr T I fully agree! I will post a patch that provides correct ARCH_ZONE_DMA_BITS for s390. BTW I wonder if ARCH_ZONE_DMA_BITS can be inferred from MAX_DMA_ADDRESS, and why do we need both.@Christoph, maybe you can help me understand if there is a relationship between the two or not, or? Regards, Halil
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