Date: Fri, 24 Apr 2015 15:15:33 +0200 From: Svatopluk Kraus <onwahe@gmail.com> To: freebsd-arm@freebsd.org Subject: Re: bus_dmamap_sync() for bounced client buffers from user address space Message-ID: <CAFHCsPXiQoAkoj8Xbyup6dmJjPD8K%2BNNTtNTNjmVB=UZOPqHMQ@mail.gmail.com> In-Reply-To: <CAFHCsPXBdeJX_yB%2B6AvM6jrsWdo8%2BZsJhVd5XK90RvXBFiqWvQ@mail.gmail.com> References: <CAFHCsPXBdeJX_yB%2B6AvM6jrsWdo8%2BZsJhVd5XK90RvXBFiqWvQ@mail.gmail.com>
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Hmm, it was intented to be posted to arch. I've already fixed that.
So, I'm sorry for noise here.
Svatopluk Kraus
On Fri, Apr 24, 2015 at 3:11 PM, Svatopluk Kraus <onwahe@gmail.com> wrote:
> DMA can be done on client buffer from user address space. For example,
> thru bus_dmamap_load_uio() when uio->uio_segflg is UIO_USERSPACE. Such
> client buffer can bounce and then, it must be copied to and from
> bounce buffer in bus_dmamap_sync().
>
> Current implementations (in all archs) do not take into account that
> bus_dmamap_sync() is asynchronous for POSTWRITE and POSTREAD in
> general. It can be asynchronous for PREWRITE and PREREAD too. For
> example, in some driver implementations where DMA client buffers
> operations are buffered. In those cases, simple bcopy() is not
> correct.
>
> Demonstration of current implementation (x86) is the following:
>
> -----------------------------
> struct bounce_page {
> vm_offset_t vaddr; /* kva of bounce buffer */
> bus_addr_t busaddr; /* Physical address */
> vm_offset_t datavaddr; /* kva of client data */
> bus_addr_t dataaddr; /* client physical address */
> bus_size_t datacount; /* client data count */
> STAILQ_ENTRY(bounce_page) links;
> };
>
>
> if ((op & BUS_DMASYNC_PREWRITE) != 0) {
> while (bpage != NULL) {
> if (bpage->datavaddr != 0) {
> bcopy((void *)bpage->datavaddr,
> (void *)bpage->vaddr,
> bpage->datacount);
> } else {
> physcopyout(bpage->dataaddr,
> (void *)bpage->vaddr,
> bpage->datacount);
> }
> bpage = STAILQ_NEXT(bpage, links);
> }
> dmat->bounce_zone->total_bounced++;
> }
> -----------------------------
>
> There are two things:
>
> (1) datavaddr is not always kva of client data, but sometimes it can
> be uva of client data.
> (2) bcopy() can be used only if datavaddr is kva or when map->pmap is
> current pmap.
>
> Note that there is an implication for bus_dmamap_load_uio() with
> uio->uio_segflg set to UIO_USERSPACE that used physical pages are
> in-core and wired. See "man bus_dma".
>
> There is not public interface to check that map->pmap is current pmap.
> So one solution is the following:
>
> if (bpage->datavaddr >= VM_MIN_KERNEL_ADDRESS) {
> bcopy();
> } else {
> physcopy();
> }
>
> If there will be public pmap_is_current() then another solution is the
> following:
>
> if (bpage->datavaddr != 0) && pmap_is_current(map->pmap)) {
> bcopy();
> } else {
> physcopy();
> }
>
> The second solution implies that context switch must not happen during
> bus_dmamap_sync() called from an interrupt routine. However, IMO, it's
> granted.
>
> Note that map->pmap should be always kernel_pmap for datavaddr >=
> VM_MIN_KERNEL_ADDRESS.
>
> Comments, different solutions, or objections?
>
> Svatopluk Kraus
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