Messages in this thread | | | From | Arnd Bergmann <> | Date | Thu, 25 Aug 2022 19:17:22 +0200 | Subject | Re: [PATCH 5/4] ARM: footbridge: remove custom DMA address handling |
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On Thu, Aug 25, 2022 at 12:35 PM Russell King (Oracle) <linux@armlinux.org.uk> wrote: > > On Thu, Aug 25, 2022 at 11:17:52AM +0200, Arnd Bergmann wrote: > > -/* > > - * The footbridge is programmed to expose the system RAM at 0xe0000000. > > - * The requirement is that the RAM isn't placed at bus address 0, which > > - * would clash with VGA cards. > > - */ > > -#define BUS_OFFSET 0xe0000000 > > -#define __virt_to_bus(x) ((x) + (BUS_OFFSET - PAGE_OFFSET)) > > -#define __bus_to_virt(x) ((x) - (BUS_OFFSET - PAGE_OFFSET)) > > ... and this tells you why that is with ISA devices. > > So, one hell of a big NAK to this patch, sorry.
Thanks for taking a look. Apparently I misread the interface, and I assumed that the __bus_to_virt() BUS_OFFSET in set_dma_addr() would just cancel out the dma_map_single() in isa_enable_dma().
It turns out the logic is actually simpler and we just convert between virtual address and bus address three times. for most drivers.
I found one driver (cs89x0-isa) that uses isa_virt_to_bus()/virt_to_phys() instead of dma_map_single(), but that is probably never used here. Once we move the DMA offset into the device, all other ISA devices will also get the default zero-offset, but I think that will be fine as long as the "isa_dma_dev" that is used for the final translation has the correct offset.
I'll try to come up with a new version tomorrow.
Arnd
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