[U-Boot] [PATCH v2 2/5] ehci-hcd: Boost transfer speed

Benoît Thébaudeau
2012-07-31T01:06:00Z

Dear Marek Vasut,

On Tue, Jul 31, 2012 at 12:38:54 AM, Marek Vasut wrote:
> [...]
>
> > > > Can you explain where this gain would come from? In both cases,
> > > > the
> > > > data in USB
> > > > transfers would be organized in the same way, and it would be
> > > > accessed in memory
> > > > also in the same way (regarding bursts). The only difference
> > > > would
> > > > be the fetch
> > > > time of a little bit more qTDs, which is extremely fast and
> > > > insignificant
> > > > compared to the transfer time of the payload, which remains
> > > > unchanged.
> > >
> > > You are right, the speed different will be minimal, only the
> > > memory
> > > usage will be lower.
> >
> > If your point is only the memory gain, I agree. With your
> > suggestion, there
> > are roughly 25% less qTDs used in the "(max wMaxPacketSize)-aligned
> > but
> > not page-aligned" case since the number of qTDs is about (total
> > transfer
> > size) / 5 instead of (total transfer size) / 4. But this is still
> > small
> > compared to usual heap sizes (at least on the kind of hardware I
> > use).
>
> Ok, I see the point. I understand it's not really a bug, just an
> improvement.

Exactly.

> Maybe we can do a subsequent patch on top of these from Benoit and
> see how it
> fares?

If you wish. I'll do that.

> > > > Moreover, in your use case, if you are e.g. using FAT, on the
> > > > one
> > > > hand, the
> > > > buffers in fat.c are never aligned to more than the DMA min
> > > > alignment, and on
> > > > the other hand, if you can align your user buffers to 512
> > > > bytes,
> > > > you can also
> > > > align them directly to 4 kB.
> > >
> > > The user buffer is aligned to 4kB, but this doesn't matter as a
> > > file
> > > load from a storage device (ex. fatload) can be segmented in
> > > partial
> > > USB
> > > transfers. This can lead to any block aligned buffer for a
> > > partial
> > > transfer.
> >
> > What do you mean by "partial USB transfers"? As seen from EHCI
> > users like
> > the MSC driver (usb_storage.c), USB transfers either succeed or
> > fail, but
> > they cannot be "segmented".
>
> Segmented -- like multiple transfers being issues with small payload?
> You can
> not put these together at the USB-level, since it's the issuing code
> that has to
> be fixed.
>
> > On its side, the MSC driver will only segment the FAT layer
> > requests if
> > they are larger than 65535 blocks, so still not what you describe.
> >
> > As to the FAT stack, it will only read whole clusters while
> > accessing file
> > payload, and the most usual cluster sizes are by default a multiple
> > of 4
> > kiB (see http://support.microsoft.com/kb/140365).
>
> 512b is minimum and it's quite often used.

OK.

> > So I don't see "segmentation" anywhere, and for usual cluster
> > sizes, the
> > EHCI buffer alignment is fully determined by the applicative buffer
> > alignment and the file position corresponding to the beginning of
> > the
> > applicative buffer. But there are indeed some unusual use cases
> > (e.g.
> > smaller clusters) for which only a block-aligned buffer will reach
> > EHCI
> > despite a page-aligned applicative buffer.
>
> I don't quite get this one.

I meant that 512 bytes (most usual storage block size) is what we should aim at
to optimize the number of qTDs.

> > > >> My suggestion would be to truncate the xfr_bytes with the max
> > > >> wMaxPacketSize (1024) and for the qtd_count use:
> > > >>
> > > >> if ((uint32_t)buffer & 1023) /* wMaxPacketSize unaligned */
> > > >>
> > > >> qtd_count += DIV_ROUND_UP(((uint32_t)buffer & 4095) +
> > > >>
> > > >> length, (QT_BUFFER_CNT - 1) * 4096);
> > > >>
> > > >> else /* wMaxPacketSize aligned */
> > > >>
> > > >> qtd_count += DIV_ROUND_UP(((uint32_t)buffer & 4095) +
> > > >>
> > > >> length, QT_BUFFER_CNT * 4096);
> > > >>
> > > >> This allows 50% of unaligned block data (512) to be
> > > >> transferred
> > > >> with
> > > >> min
> > > >> qTDs.
> > > >
> > > > That would also require a realignment-to-page stage. This is
> > > > specific code for
> > > > specific buffer alignment from the upper layers. We could also
> > > > skip
> > > > the
> > > > realignment to page and always keep the same qTD transfer size
> > > > except for the
> > > > last one, by adding as many packets as possible for the buffer
> > > > alignment.
> > >
> > > What you mean by realignment-to-page stage?
> >
> > I mean that the alignment of the transfer to 1024 instead of 4096
> > can make
> > the first qTD transfer larger than the following ones, which
> > guarantees
> > that the following qTD transfers are page-aligned, even if the
> > first one
> > was only aligned to 1024. For the 1024-aligned case, this results
> > in the
> > change that you suggest, but it also changes things for the
> > unaligned
> > case, which makes this part of the code inaccurate. See below.
> >
> > > > But I still don't see a significant reason to complicate code
> > > > to do
> > > > that.
> > >
> > > I don't understand where you expect to complicate the code.
> > >
> > > You limit the size of one transfer (xfr_bytes) to (QT_BUFFER_CNT
> > > - 1)
> > > *
> > > 4kB for unaligned buffers. But you only need to limit it to a
> > > multiple
> > > of the maximal possible wMaxPacketSize (1kB) to make sure that
> > > you
> > > always send full packages.
> > >
> > > I only suggest to replace the causeless 4kB alignment with a
> > > reasonable
> > > 1kB alignment and adapte the qtd_count caluclation.
> > >
> > > int xfr_bytes = min(left_length,
> > >
> > > (QT_BUFFER_CNT *
> > > 4096 -
> > >
> > > ((uint32_t)buf_ptr
> > > &
> > > 4095)) &
> > >
> > > - ~4095);
> > > + ~1023);
> >
> > I agree for this part of the code. But for the allocation part,
> > your
> > suggestion is already a little bit more complicated than my
> > original code,
> > while still incomplete. Besides that, the "((uint32_t)buffer &
> > 4095) +"
> > for the page-unaligned case in my code was actually useless, which
> > emphasizes the difference, even if it's a light complication.
> >
> > For the allocation part, the appropriate code for your suggestion
> > would be:
> >
> > if ((uint32_t)buffer & 1023) /* max-wMaxPacketSize-unaligned */
> > qtd_count +=
>
> DIV_ROUND_UP(
> max(
> length > 0,
> length - (4096 - ((uint32_t)buf_ptr & 4095) & ~1023)
> ),
> (QT_BUFFER_CNT - 1) * 4096
> );
>
> Ok, I now think I understand what's going on here. I still have to
> wonder how
> much would the compiler optimize of this if you "decompressed" your
> code -- to
> make it more easy to understand.

I wouldn't go for this complicated version since it's not really useful compared
to the simpler yet less accurate solution I gave below.

> > else /* max-wMaxPacketSize-aligned */
> > qtd_count += DIV_ROUND_UP(((uint32_t)buffer & 4095) +
> > length, QT_BUFFER_CNT * 4096);
> >
> > This code allocates exactly the required number of qTDs, no less,
> > no more.
> > It's clearly more complicated than the 4096 version.
> >
> > A test should also be added to make sure that qtd_count is never 0.
> > Otherwise, ZLPs are broken (this applies to my original code too).
> >
> > If we want to compromise accuracy for simplicity, we can change
> > that to:
> >
> > qtd_count += 2 + length /
> > ((QT_BUFFER_CNT - !!((uint32_t)buffer & 1023)) * 4096);

It's this solution I'd like to use to optimize the number of qTDs (with 1023 or
something else).

> > This code allocates enough qTDs for all cases, with at worst 2
> > extra qTDs
> > (i.e. a total of 128 bytes) that will be left unused. It also
> > handles
> > intrinsically ZLPs.
> >
> > Now, let's consider the possible values of wMaxPacketSize:
> > - control endpoints:
> > * LS: 8,
> > * FS: 8, 16, 32 or 64,
> > * HS: 64,
> > - isochronous endpoints: not supported by ehci-hcd.c,
> > - interrupt endpoints:
> > * LS: <= 8,
> > * FS: <= 64,
> > * HS: <= 1024 (1x to 3x for high-bandwidth),
> > - bulk endpoints:
> > * LS: N/A,
> > * FS: 8, 16, 32 or 64,
> > * HS: 512.
> >
> > My code assumes that wMaxPacketSize is a power of 2. This is not
> > always
> > true for interrupt endpoints. Let's talk about these. Their
> > handling is
> > currently broken in U-Boot since their transfers are made
> > asynchronous
> > instead of periodic. Devices shouldn't care too much about that, as
> > long
> > as transfers do not exceed wMaxPacketSize, in which case my code
> > still
> > works because wMaxPacketSize always fits in a single qTD. Interrupt
> > transfers larger than wMaxPacketSize do not seem to be used by
> > U-Boot. If
> > they were used, the current code in U-Boot would have a timing
> > issue
> > because the asynchronous scheme would break the interval requested
> > by
> > devices, which could at worst make them fail in some way. So the
> > only
> > solution would be that such transfers be split by the caller of
> > submit_int_msg, in which case my code still works. What would you
> > think
> > about failing with an error message in submit_int_msg if length is
> > larger
> > than wMaxPacketSize? Marek, what do you think?
>
> Let's do that ... I think the interrupt endpoint is only used for
> keyboard and
> if someone needs it for something else, the code will be there, just
> needing
> improvement. Comment and error message are OK.

OK. I have thought of another solution for this. You'll tell me which one you
prefer.

The ehci_submit_async code currently in U-Boot checks through ehci_td_buffer
that length fits in the single qTD reserved for data payload only after work has
begun, possibly after a SETUP transfer. With my series, this is checked at the
very beginning, before the allocation. We could detect that wMaxPacketSize is
not a power of 2 (e.g. with __builtin_popcount), in which case the allocation
for the data payload would be restricted to 1 qTD like now, and there would be
a check at the very beginning to test if length fits in this qTD. In that way,
there could be several packets per interrupt transfer as long as it fits in a
single qTD, just like now, contrary to the limitation imposed by the error in
submit_int_msg. But I'm not sure it's a good idea to allow this behavior.

> > For all other cases, wMaxPacketSize is a power of 2, so everything
> > is fine,
> > except that in those cases wMaxPacketSize is at most 512, which
> > means that
> > with the suggested limitation applied to submit_int_msg, your
> > suggested
> > 1024 could be replaced with 512, which is good news since this is
> > also the
> > most common storage sector size.
> >
> > We could even use usb_maxpacket(dev, pipe) instead of 512, with
> > some
> > restrictions. If we don't want to alter the misalignment check in
> > ehci_td_buffer, max(usb_maxpacket(dev, pipe), ARCH_DMA_MINALIGN)
> > would
> > actually have to be used. This misalignment check could be limited
> > to the
> > first qTD transfer of a USB transfer, but that would complicate
> > things,
> > all the more the corresponding call to flush_dcache_range would
> > have to be
> > modified to fix alignments.
> >
> > So we have to make two choices:
> > - between 4096, 1024, 512 and max(usb_maxpacket(dev, pipe),
> > ARCH_DMA_MINALIGN), - between the accurate and simple allocations.
> > That makes a total of 8 working possibilities. What do you guys
> > think we
> > should choose? On my side, I like max(usb_maxpacket(dev, pipe),
> > ARCH_DMA_MINALIGN)
>
> Won't maxpacket fall below 512 on occasions,

Sure.

> which might cause
> trouble?

Why?

> > with the simple allocation. It's efficient as to code
> > speed, size and readability, as well as to RAM usage.
>
> For now, I'd go for the safest, easiest and dumbest solution and see
> how it
> fares. Subsequent patch can be submitted to improve that and
> measurements made.
>
> "We should forget about small efficiencies, say about 97% of the
> time; premature
> optimization is the root of all evil"
> -- Donald E. Knuth, Structured Programming with go to
> Statements
> [...]

OK, so I'll stick to my original series, rework it lightly as we said, add Jim's
patch, and add a further patch for these optimizations.

> > So we could perhaps issue a #error in ehci-hcd or in usb_storage if
> > CONFIG_SYS_MALLOC_LEN is not large enough, but I don't think it's a
> > good
> > idea because:
> > - the threshold value would have to depend on runtime block sizes
> > or
> > something, which could lead to a big worst worst case that would
> > almost
> > never happen in real life, so giving such an unrealistic heap size
> > constraint would be cumbersome,
>
> #warning then?

With which limit if so?

> > - reaching the top sizes would mean reading a huge file or
> > something to a
> > large buffer (much larger than the qTDs this transfer requires),
> > which
> > would very likely be heap-allocated (except for commands like
> > fatload), so
> > CONFIG_SYS_MALLOC_LEN would already have to be large for the
> > application,
> > - for command line operations like fatload, transfers of
> > uncontrolled
> > lengths could simply crash U-Boot if they go too far in memory
>
> Why, because they overwrite it?

Yes. U-Boot expands down its allocation during startup, so it's often located at
the end of the embedded RAM, which means that fatload will very likely use the
beginning of the RAM.

> > , which
> > means that users of such commands need to know what they are doing
> > anyway,
> > so they have to control transfer sizes,
> > - there is already a runtime error displayed in case of allocation
> > failure.
>
> Ok

So #warning or not besides this?

> > Marek, what do you think?
>
> Had a good evening with the EHCI r10 spec, hope I answered most of
> your
> questions.

Yes, thanks.

Best regards,
Beno?t