From mboxrd@z Thu Jan 1 00:00:00 1970 From: vladimir.murzin@arm.com (Vladimir Murzin) Date: Wed, 11 Jan 2017 14:34:58 +0000 Subject: [RFC PATCH v4 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU In-Reply-To: References: <1484057925-23586-1-git-send-email-vladimir.murzin@arm.com> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 11/01/17 13:17, Benjamin Gaignard wrote: > 2017-01-10 15:18 GMT+01:00 Vladimir Murzin : >> Hi, >> >> It seem that addition of cache support for M-class cpus uncovered >> latent bug in DMA usage. NOMMU memory model has been treated as being >> always consistent; however, for R/M classes of cpu memory can be >> covered by MPU which in turn might configure RAM as Normal >> i.e. bufferable and cacheable. It breaks dma_alloc_coherent() and >> friends, since data can stuck in caches now or be buffered. >> >> This patch set is trying to address the issue by providing region of >> memory suitable for consistent DMA operations. It is supposed that >> such region is marked by MPU as non-cacheable. Robin suggested to >> advertise such memory as reserved shared-dma-pool, rather then using >> homebrew command line option, and extend dma-coherent to provide >> default DMA area in the similar way as it is done for CMA (PATCH >> 2/5). It allows us to offload all bookkeeping on generic coherent DMA >> framework, and it is seems that it might be reused by other >> architectures like c6x and blackfin. >> >> Dedicated DMA region is required for cases other than: >> - MMU/MPU is off >> - cpu is v7m w/o cache support >> - device is coherent >> >> In case one of the above conditions is true dma operations are forced >> to be coherent and wired with dma_noop_ops. >> >> To make life easier NOMMU dma operations are kept in separate >> compilation unit. >> >> Since the issue was reported in the same time as Benjamin sent his >> patch [1] to allow mmap for NOMMU, his case is also addressed in this >> series (PATCH 1/5 and PATCH 3/5). >> >> Thanks! > > I have tested this v4 on my setup (stm32f4, no cache, no MPU) and unfortunately > it doesn't work with my drm/kms driver. I guess the same is for fbmem, but would be better to have confirmation since amba-clcd I use has not been ported to drm/kms (yet), so I can't test. > I haven't any errors but nothing is displayed unlike what I have when > using current dma-mapping > code. > I guess the issue is coming from dma-noop where __get_free_pages() is > used instead of alloc_pages() > in dma-mapping. Unless I've missed something bellow is a call stack for both #1 __alloc_simple_buffer __dma_alloc_buffer alloc_pages split_page __dma_clear_buffer memset page_address #2 __get_free_pages alloc_pages page_address So the difference is that nommu case in dma-mapping.c memzeros memory, handles DMA_ATTR_NO_KERNEL_MAPPING and does optimisation of memory usage. Is something from above critical for your driver? > > Since my hardware doesn't have cache or MPU (and so use dma-noop) I > haven't reserved specific memory region. > Buffer addresses and vma parameters look correct... What could I have > miss here ? No ideas, sorry... Cheers Vladimir > > Benjamin > >> >> [1] http://www.armlinux.org.uk/developer/patches/viewpatch.php?id=8633/1 >> >> Vladimir Murzin (5): >> dma: Add simple dma_noop_mmap >> drivers: dma-coherent: Introduce default DMA pool >> ARM: NOMMU: Introduce dma operations for noMMU >> ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus >> ARM: dma-mapping: Remove traces of NOMMU code >> >> .../bindings/reserved-memory/reserved-memory.txt | 3 + >> arch/arm/include/asm/dma-mapping.h | 3 +- >> arch/arm/mm/Kconfig | 2 +- >> arch/arm/mm/Makefile | 5 +- >> arch/arm/mm/dma-mapping-nommu.c | 252 +++++++++++++++++++++ >> arch/arm/mm/dma-mapping.c | 26 +-- >> drivers/base/dma-coherent.c | 59 ++++- >> lib/dma-noop.c | 21 ++ >> 8 files changed, 335 insertions(+), 36 deletions(-) >> create mode 100644 arch/arm/mm/dma-mapping-nommu.c >> >> -- >> 2.0.0 >> > > >