Date: Tue, 27 Nov 2018 14:24:44 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Rajesh Kumar <rajfbsd@gmail.com> Cc: freebsd-drivers@freebsd.org, freebsd-hackers@freebsd.org Subject: Re: bus_dmamem_alloc doesn't follow alignment specified Message-ID: <20181127122444.GB2378@kib.kiev.ua> In-Reply-To: <CAAO%2BANMNFv5UBG1H7hOziW1DWVMGNTRp4ZQcGCF29XS-SwpG7g@mail.gmail.com> References: <CAAO%2BANMNFv5UBG1H7hOziW1DWVMGNTRp4ZQcGCF29XS-SwpG7g@mail.gmail.com>
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On Tue, Nov 27, 2018 at 04:37:57PM +0530, Rajesh Kumar wrote:
> Hi,
>
> I am writing a PCI-E driver, where I am trying to allocate DMA buffers. But
> seems, like the alignment requirements are not satisfied by
> bus_dmamem_alloc function. I am using stable/11 branch.
>
> *Code :*
> dma_setup(size, align, alimit) {
>
> printf ("size 0x%x align 0x%x alimit 0x%x\n", size, align, alimit);
>
> if (bus_dma_tag_create(parent, align, 0,
> alimit, BUS_SPACE_MAXADDR,
> NULL, NULL, size, 1, size,
> 0, NULL, NULL, &dma_tag)) {
> return (ENOMEM);
> }
>
> if (bus_dmamem_alloc(dma_tag, &virt_addr,
> BUS_DMA_WAITOK | BUS_DMA_ZERO, &dma_map)) {
> bus_dma_tag_destroy(dma_tag);
> return (ENOMEM);
> }
>
> if (bus_dmamap_load(dma_tag, dma_map, virt_addr,
> size, dma_cbfn, &arg, BUS_DMA_NOWAIT)) {
> bus_dmamem_free(dma_tag, virt_addr, dma_map);
> bus_dma_tag_destroy(mw->dma_tag);
> return (ENOMEM);
> }
>
> printf("dma_addr 0x%lx virt_addr 0x%lx\n", (uint64_t)arg->addr,
> (uint64_t)virt_addr);
>
> }
>
> dma_cbfn(void *tmp, bus_dma_segment_t *segs, int nsegs, int error)
> {
> struct dma_args *arg = (struct dma_args *)tmp;
> arg->addr = segs[0].ds_addr;
> }
>
> *Logs: *
> size 0x100000 align 0x100000 alimit 0xffffffff
> dma_addr *0x78e00000* virt_addr *0xfffffe1c3665f000*
>
> As seen above, dma_addr is as per alignment requirement, but virtual
> address from bus_dmamem_alloc doesn't seem to satisfy the alignment
> requirement).
>
> My understanding is both the DMA addr and the virtual address should be as
> per the alignment specified. Can anyone please clarify?
Only bus address is aligned.
For single-segment i.e. contiguous allocations, amd64 could benefit from
using dmap VA instead of kmem_alloc. This would be only an optimization,
not done right now.
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