Re: [RFC PATCH 0/4] spi: cadence-xspi: add ACMD PIO support for NAND and NOR

["Nuno Sá" <noname.nuno@gmail.com>]

On Fri, 2026-10-09 at 12:30 +0800, Jisheng Zhang wrote:
> On Thu, Oct 08, 2026 at 09:55:27PM +0800, Jisheng Zhang wrote:
> > On Wed, Oct 07, 2026 at 07:12:57AM +0100, Nuno Sá wrote:
> > > On Wed, 2026-10-07 at 12:48 +0800, Jisheng Zhang wrote:
> > > > On Tue, Oct 06, 2026 at 07:50:44AM +0100, Nuno Sá wrote:
> > > > > On Sun, 2026-10-04 at 16:37 +0200, Miquel Raynal wrote:
> > > > > > Hello,
> > > > > > 
> > > > > > On 28/09/2026 at 16:02:27 +01, Nuno Sá <nuno.sa@analog.com> wrote:
> > > > > > 
> > > > > > > On Mon, Sep 28, 2026 at 01:50:05PM +0800, Fei Xie wrote:
> > > > > > > 
> > > > > > > Hi Fei,
> > > > > > > 
> > > > > > > > Hi Nuno, Miquel,
> > > > > > > > 
> > > > > > > > Thanks for asking for benchmarks. I backported the 32/64-bit STIG
> > > > > > > > SDMA
> > > > > > > > transfer changes from the patch Nuno pointed to [1] to our vendor 6.1
> > > > > > > > kernel, and compared that STIG path against the ACMD SPI NAND path
> > > > > > > > from
> > > > > > > > this RFC. I have not applied Nuno's separate ACMD read
> > > > > > > > implementation;
> > > > > > > > the figures below are for the implementation in this RFC.
> > > > > > > > 
> > > > > > > > The device is a GD5F4GM8RE SPI NAND (2 KiB pages, 128 KiB
> > > > > > > > eraseblocks)
> > > > > > > > on a Horizon J6B board, running at 80 MHz in quad SDR mode. Both
> > > > > > > > paths
> > > > > > > > used the same board PHY configuration. STIG needed one additional
> > > > > > > > dummy
> > > > > > > > clock for the EBh read-cache command on this board to return correct
> > > > > > > > data at 80 MHz. This is a local test adjustment, not part of the
> > > > > > > > posted
> > > > > > > > RFC or [1]; I excluded the incorrect STIG reads from the comparison.
> > > > > > > > 
> > > > > > > > I ran flash_speed -c 5 -d /dev/mtd18 three times with each mode. The
> > > > > > > > numbers below are the arithmetic means, in KiB/s:
> > > > > > > > 
> > > > > > > >                                STIG      ACMD PIO
> > > > > > > >   eraseblock write speed      3685          3555
> > > > > > > >   eraseblock read speed      12468         14222
> > > > > > > >   page write speed            3671          3510
> > > > > > > >   page read speed            11931         13716
> > > > > > > >   erase speed                40000         37647
> > > > > > > 
> > > > > > > So this goes along with my expectations. I think we can all agree that
> > > > > > > PROGRAM + ERASE is not worth the trouble? We even loose performance...
> > > > > > 
> > > > > > Yeah.
> > > > > > 
> > > > > > > > For a separate sequential-read test, I erased and programmed the
> > > > > > > > first
> > > > > > > > 64 MiB with known random data, then checked that each mode read back
> > > > > > > > the complete area byte-for-byte. Both SHA-256 hashes matched the
> > > > > > > > source;
> > > > > > > > the reported ECC failure counts were zero. I then ran:
> > > > > > > 
> > > > > > > What do you mean by sequential read? Does your flash supports
> > > > > > > continuous
> > > > > > > rads (or buffered mode I think). If so, I would expect much better
> > > > > > > number for the ACMD case. But that can be the issue of having both the
> > > > > > > controller and the chip trying to walk pages.
> > > > > > 
> > > > > > Sequential, continuous and buffered should mean the same in the NAND
> > > > > > context.
> > > > > > 
> > > > > > > >   /var/busybox/time -f 'real=%e user=%U sys=%S cpu=%P' \
> > > > > > > >     dd if=/dev/mtd18 of=/dev/null bs=1M count=64
> > > > > > > > 
> > > > > > > >                           STIG              ACMD PIO
> > > > > > > >   elapsed (three runs)    5.30/5.28/5.26 s  4.66/4.65/4.65 s
> > > > > > > >   mean throughput         12.12 MiB/s       13.75 MiB/s
> > > > > > > >   mean system time        3.55 s            1.06 s
> > > > > > > >   process CPU usage       67%               22%
> > > > > > > > 
> > > > > > > > On this NAND, ACMD improves the 64 MiB read throughput by about 13.5%
> > > > > > > > and reduces the reading process's CPU usage by 45 percentage points.
> > > > > > > > The flash_speed results show a similar gain for reads, including
> > > > > > > > single-page reads.
> > > > > > 
> > > > > > FYI, if you pull a recent version of mtd-utils, flash_speed can
> > > > > > benchmark continuous read improvements with the -C option.
> > > > > > 
> > > > > > > > I do not see a program or erase throughput benefit:
> > > > > > > > ACMD is slightly slower for both in this setup. These are SPI NAND
> > > > > > > > results only; I have not benchmarked the NOR path on this board.
> > > > > > > 
> > > > > > > So CPU results are the bing thing here. I guess we need to decide if
> > > > > > > it's worth the amount of complexity it requires for the whole thing to
> > > > > > > work. As I mentioned, if the chip has continuous mode and we try to
> > > > > > > read anything bigger than page size we should let the nand chip to
> > > > > > > walk the pages and that means the behavior we have today (IMHO of
> > > > > > > course):
> > > > > > > 
> > > > > > > 0x13 (STIG) + wait + read from cache (ACMD)
> > > > > > > 
> > > > > > > If we're only reading a single page then the logic in this series would
> > > > > > > payoff (slight performance gain + significant drop in cpu load). One
> > > > > > > simple
> > > > > > > way for doing this as a starting point would be:
> > > > > > > 
> > > > > > > * Nand has cont mode -> xspi controller does nothing special
> > > > > > > * Nand has no cont mode -> Aggregate the xfers together.
> > > > > > 
> > > > > > I'm fine with static decisions like that, but my understanding of the
> > > > > > above is that ACMD mode is useful for all reads. so I don't get why you
> > > > > > would no use it for all. Maybe because it would break continuous reads?
> > > > > > (sorry I lost a bit of the context already).
> > > > > 
> > > > > Exactly. In ACMD, is also capable of "walking pages" for reads bigger than
> > > > > page
> > > > > size
> > > > > (hence the need to know the flash geometry) and IIRC, that does not really
> > > > > work
> > > > > well
> > > > > when the flash is also doing it.
> > > > > 
> > > > > > 
> > > > > > For writes and erases however, it seems like we could keep the STIG path.
> > > > > > 
> > > > > > > Doing something dynamic would be much more complex I guess. With the
> > > > > > > above reasoning, having some mem_info() kind of spi_mem callback to
> > > > > > > tell
> > > > > > > us things like geometry, continuous mode support, etc would be enough
> > > > > > > to
> > > > > > > handle the above I think. The reasoning is that reading a nand page is
> > > > > > > always like spinand_load_page_op() (opcode 0x13) + spinand_wait() +
> > > > > > > spinand_read_from_cache_op() so we could probably aggregate everything
> > > > > > > in the last call on the controller side without complicating the nand
> > > > > > > core
> > > > > > > logic.
> > > > > > 
> > > > > > In general, drivers making assumptions has been a disaster in the raw
> > > > > > NAND world. This was the main reason for the ->exec_op() introduction
> > > > > > there: giving the whole operation to the controller so it can decide
> > > > > > what to do without guessing. But we thinking at the operation boundary,
> > > > > > not at the "set of operations" level. Since ->exec_op() in the SPI NAND
> > > > > > subsystem has been written following the same philosophy, I fear we're
> > > > > > going to try make rounds fit inside squares. One of the possible
> > > > > 
> > > > > Fair enough! Agreed
> > > > > 
> > > > > > approach would be to mimic what we've done for the some limited
> > > > > > controllers: pack everything in one single op (see the _monolithic
> > > > > > suffix in drivers/mtd/nand/raw/). That's just one idea.
> > > > > > 
> > > > > 
> > > > > I can take a look mostly for curiosity as my intention is not to "steal"
> > > > > this
> > > > > work
> > > > > from Fei. If everyone agrees (specially Fei), what I can do is to send my
> > > > > simpler
> > > > > patches that just support ACMD (without any special distinction between
> > > > > Nand and
> > > > > Nor)
> > > > > For Nands, I have a mix of STIG and ACMD, where big gains come from Nands
> > > > > with
> > > > > cont
> > > > > mode.
> > > > > 
> > > > > Then we can build on top of it in order to improve Nand support (mostly on
> > > > > CPU
> > > > > load).
> > > > 
> > > > I'm also interested in this driver's performance. A bit off the subject
> > > > but related question: Usually, the Soc may connect the xspi to dma
> > > > engine, I'm curious what would be the benefit of adding dma engine
> > > > support instead of ACMD? Is dma engine support simpler and
> > > > is the gain(performance + cpu load) more?
> > > > 
> > > 
> > > I'm not familiar with that approach tbh. But I would assume performance to be
> > > pretty
> > > much similar to what I have given that ACMD uses the IP internal DMA engine.
> > > But not
> > > using ACMD means not having the cpu gains at least for the smaller transfers as
> > > we
> > > would not be able to aggregate transfers. FWIW, given that you might not have
> > > looked
> > 
> > Hmm, my platforms connect two dmaengine channels to the xspi controller,
> > but I haven't added the dma-engine support to the driver yet. If the
> > ACMD is enough, why HW connects dma-engine then? Just curious, there
> > must be performance gain I guess. But I'm not sure and I don't have
> > performance numbers.
> 

SO the IP has an internal DMA engine capable of acting as controller and driving the
bus AFAIU. So we do not really need any external dma_chan. Just need to use the dma
mapping API,

> Aha, my platforms don't enable the ACMD support, instead, connect the
> xspi with dmac.
> 
> So when upstreaming ACMD support, the driver needs to detect the ACMD
> support either dynamically or via. DT. Unfortunately, I didn't find
> any register can export the ACMD availability. DT is the only way?
> Or issue a simple ACMD and check SDMA error?

I'm really not sure on your usecase. ACMD starts to payoff really when the transfers
start to be big enough. For example for my usecase, Im only using ACMD for reads
bigger than > 2KiB. That number is fixed but I suspect it's not that simple given
that It might depend on number of lanes and clock speeds.

If I'm not missing anything, the easiest way to handle things would be to detect
dma_chan from DT and if available use it instead of ACMD.


- Nuno Sá

> > 
> > > at the whole thread, this is what I have:
> > > 
> > > https://github.com/analogdevicesinc/linux/pull/3478/changes/477e6095508546552071b825a66522cf697c27c2
> > > 
> > > - Nuno Sá
> > > 
> > > > Regards
> 
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