On Mon, May 23, 2022 at 04:09:20PM +0000, David Laight wrote: > From: Petr Malat > > Sent: 23 May 2022 16:28 > > On Mon, May 23, 2022 at 02:51:41PM +0000, David Laight wrote: > > > From: Petr Malat > > > > Sent: 23 May 2022 15:28 > > > > > > > > One can't use memcpy on memory obtained by ioremap, because IO memory > > > > may have different alignment and size access restriction than the system > > > > memory. Use memremap as phram driver operates on RAM. > > > > > > Does that actually help? > > > The memcpy() is still likely to issue unaligned accesses > > > that the hardware can't handle. > > > > Yes, it solves the issue. Memcpy can cause unaligned access only on > > platforms, which can handle it. And on ARM64 it's handled only for > > RAM and not for a device memory (__pgprot(PROT_DEVICE_*)). > > Does mapping it as memory cause it to be cached? > So the hardware only sees cache line reads (which are aligned) > and the cpu support for misaligned memory accesses then > stop the faults? Yes, this is controlled by the MEMREMAP_WB flag, which sets up a mapping, which "matches the default mapping for System RAM on the architecture. This is usually a read-allocate write-back cache. > On x86 (which I know a lot more about) memcpy() has a nasty > habit of getting implemented as 'rep movsb' relying on the > cpu to speed it up. > But that doesn't happen for uncached addresses - so you get > very slow byte copies. > OTOH misaligned PCIe transfers generate TLP that have the > correct byte enables for the end words. > Provided the PCIe target isn't broken they are fine. With memremap one should get the same behavior and performance as with the system memory and it seems to be a good choice for "Physical system RAM" MTD driver, but if one uses it for actual IO memory, he should use ioremam, memcpy_toio and memcpy_fromio. Using these prevents the crash on arm64 as well, but could lead to a performance degradation on some platforms. If you think there could be users using the driver for a real IO mem, I can provide both behaviors and let the user choose with an option. Petr
