[PATCH v3 net-next 0/2] Adapt the sja1105 DSA driver to the SPI controller's transfer limits

[PATCH v3 net-next 0/2] Adapt the sja1105 DSA driver to the SPI controller's transfer limits

[Vladimir Oltean]

From: Vladimir Oltean <vladimir.oltean@nxp.com>

This series changes the SPI transfer procedure in sja1105 to take into
consideration the buffer size limitations that the SPI controller driver
might have.

Changes in v3:
- Avoid a signed vs unsigned issue in the interpretation of SIZE_MAX.
- Move the max transfer length checks to probe time, since nothing will
  change dynamically.

Changes in v2:
Remove the driver's use of cs_change and send multiple, smaller SPI
messages instead of a single large one.

Vladimir Oltean (2):
  net: dsa: sja1105: send multiple spi_messages instead of using
    cs_change
  net: dsa: sja1105: adapt to a SPI controller with a limited max
    transfer size

 drivers/net/dsa/sja1105/sja1105.h             |  1 +
 drivers/net/dsa/sja1105/sja1105_main.c        | 28 ++++++++
 drivers/net/dsa/sja1105/sja1105_spi.c         | 66 +++++--------------
 .../net/dsa/sja1105/sja1105_static_config.h   |  2 +
 4 files changed, 49 insertions(+), 48 deletions(-)

-- 
2.25.1

[PATCH v3 net-next 1/2] net: dsa: sja1105: send multiple spi_messages instead of using cs_change

[Vladimir Oltean]

From: Vladimir Oltean <vladimir.oltean@nxp.com>

The sja1105 driver has been described by Mark Brown as "not using the
[ SPI ] API at all idiomatically" due to the use of cs_change:
https://patchwork.kernel.org/project/netdevbpf/patch/20210520135031.2969183-1-olteanv@gmail.com/

According to include/linux/spi/spi.h, the chip select is supposed to be
asserted for the entire length of a SPI message, as long as cs_change is
false for all member transfers. The cs_change flag changes the following:

(i) When a non-final SPI transfer has cs_change = true, the chip select
    should temporarily deassert and then reassert starting with the next
    transfer.
(ii) When a final SPI transfer has cs_change = true, the chip select
     should remain asserted until the following SPI message.

The sja1105 driver only uses cs_change for its first property, to form a
single SPI message whose layout can be seen below:

                                             this is an entire, single spi_message
           _______________________________________________________________________________________________
          /                                                                                               \
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
          | hdr_xfer[0] | chunk_xfer[0] | hdr_xfer[1] | chunk_xfer[1] |     | hdr_xfer[n] | chunk_xfer[n] |
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
cs_change      false          true           false           true                false          false

           ____________________________  _____________________________       _____________________________
CS line __/                            \/                             \ ... /                             \__

The fact of the matter is that spi_max_message_size() has an ambiguous
meaning if any non-final transfer has cs_change = true.

If the SPI master has a limitation in that it cannot keep the chip
select asserted for more than, say, 200 bytes (like the spi-sc18is602),
the normal thing for it to do is to implement .max_transfer_size and
.max_message_size, and limit both to 200: in the "worst case" where
cs_change is always false, then the controller can, indeed, not send
messages larger than 200 bytes.

But the fact that the SPI controller's max_message_size does not
necessarily mean that we cannot send messages larger than that.
Notably, if the SPI master special-cases the transfers with cs_change
and treats every chip select toggling as an entirely new transaction,
then a SPI message can easily exceed that limit. So there is a
temptation to ignore the controller's reported max_message_size when
using cs_change = true in non-final transfers.

But that can lead to false conclusions. As Mark points out, the SPI
controller might have a different kind of limitation with the max
message size, that has nothing at all to do with how long it can keep
the chip select asserted.
For example, that might be the case if the device is able to offload the
chip select changes to the hardware as part of the data stream, and it
packs the entire stream of commands+data (corresponding to a SPI
message) into a single DMA transfer that is itself limited in size.

So the only thing we can do is avoid ambiguity by not using cs_change at
all. Instead of sending a single spi_message, we now send multiple SPI
messages as follows:

                  spi_message 0                 spi_message 1                       spi_message n
           ____________________________   ___________________________        _____________________________
          /                            \ /                           \      /                             \
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
          | hdr_xfer[0] | chunk_xfer[0] | hdr_xfer[1] | chunk_xfer[1] |     | hdr_xfer[n] | chunk_xfer[n] |
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
cs_change      false          true           false           true                false          false

           ____________________________  _____________________________       _____________________________
CS line __/                            \/                             \ ... /                             \__

which is clearer because the max_message_size limit is now easier to
enforce. What is transmitted on the wire stays, of course, the same.

Additionally, because we send no more than 2 transfers at a time, we now
avoid dynamic memory allocation too, which might be seen as an
improvement by some.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 drivers/net/dsa/sja1105/sja1105_spi.c | 52 +++++++--------------------
 1 file changed, 12 insertions(+), 40 deletions(-)

diff --git a/drivers/net/dsa/sja1105/sja1105_spi.c b/drivers/net/dsa/sja1105/sja1105_spi.c
index f7a1514f81e8..8746e3f158a0 100644
--- a/drivers/net/dsa/sja1105/sja1105_spi.c
+++ b/drivers/net/dsa/sja1105/sja1105_spi.c
@@ -29,13 +29,6 @@ sja1105_spi_message_pack(void *buf, const struct sja1105_spi_message *msg)
 	sja1105_pack(buf, &msg->address,    24,  4, size);
 }
 
-#define sja1105_hdr_xfer(xfers, chunk) \
-	((xfers) + 2 * (chunk))
-#define sja1105_chunk_xfer(xfers, chunk) \
-	((xfers) + 2 * (chunk) + 1)
-#define sja1105_hdr_buf(hdr_bufs, chunk) \
-	((hdr_bufs) + (chunk) * SJA1105_SIZE_SPI_MSG_HEADER)
-
 /* If @rw is:
  * - SPI_WRITE: creates and sends an SPI write message at absolute
  *		address reg_addr, taking @len bytes from *buf
@@ -46,41 +39,25 @@ static int sja1105_xfer(const struct sja1105_private *priv,
 			sja1105_spi_rw_mode_t rw, u64 reg_addr, u8 *buf,
 			size_t len, struct ptp_system_timestamp *ptp_sts)
 {
+	u8 hdr_buf[SJA1105_SIZE_SPI_MSG_HEADER] = {0};
 	struct sja1105_chunk chunk = {
 		.len = min_t(size_t, len, SJA1105_SIZE_SPI_MSG_MAXLEN),
 		.reg_addr = reg_addr,
 		.buf = buf,
 	};
 	struct spi_device *spi = priv->spidev;
-	struct spi_transfer *xfers;
+	struct spi_transfer xfers[2] = {0};
+	struct spi_transfer *chunk_xfer;
+	struct spi_transfer *hdr_xfer;
 	int num_chunks;
 	int rc, i = 0;
-	u8 *hdr_bufs;
 
 	num_chunks = DIV_ROUND_UP(len, SJA1105_SIZE_SPI_MSG_MAXLEN);
 
-	/* One transfer for each message header, one for each message
-	 * payload (chunk).
-	 */
-	xfers = kcalloc(2 * num_chunks, sizeof(struct spi_transfer),
-			GFP_KERNEL);
-	if (!xfers)
-		return -ENOMEM;
-
-	/* Packed buffers for the num_chunks SPI message headers,
-	 * stored as a contiguous array
-	 */
-	hdr_bufs = kcalloc(num_chunks, SJA1105_SIZE_SPI_MSG_HEADER,
-			   GFP_KERNEL);
-	if (!hdr_bufs) {
-		kfree(xfers);
-		return -ENOMEM;
-	}
+	hdr_xfer = &xfers[0];
+	chunk_xfer = &xfers[1];
 
 	for (i = 0; i < num_chunks; i++) {
-		struct spi_transfer *chunk_xfer = sja1105_chunk_xfer(xfers, i);
-		struct spi_transfer *hdr_xfer = sja1105_hdr_xfer(xfers, i);
-		u8 *hdr_buf = sja1105_hdr_buf(hdr_bufs, i);
 		struct spi_transfer *ptp_sts_xfer;
 		struct sja1105_spi_message msg;
 
@@ -129,19 +106,14 @@ static int sja1105_xfer(const struct sja1105_private *priv,
 		chunk.len = min_t(size_t, (ptrdiff_t)(buf + len - chunk.buf),
 				  SJA1105_SIZE_SPI_MSG_MAXLEN);
 
-		/* De-assert the chip select after each chunk. */
-		if (chunk.len)
-			chunk_xfer->cs_change = 1;
+		rc = spi_sync_transfer(spi, xfers, 2);
+		if (rc < 0) {
+			dev_err(&spi->dev, "SPI transfer failed: %d\n", rc);
+			return rc;
+		}
 	}
 
-	rc = spi_sync_transfer(spi, xfers, 2 * num_chunks);
-	if (rc < 0)
-		dev_err(&spi->dev, "SPI transfer failed: %d\n", rc);
-
-	kfree(hdr_bufs);
-	kfree(xfers);
-
-	return rc;
+	return 0;
 }
 
 int sja1105_xfer_buf(const struct sja1105_private *priv,
-- 
2.25.1

Re: [PATCH v3 net-next 1/2] net: dsa: sja1105: send multiple spi_messages instead of using cs_change

[Mark Brown]

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On Fri, May 21, 2021 at 12:16:56AM +0300, Vladimir Oltean wrote:

> The fact of the matter is that spi_max_message_size() has an ambiguous
> meaning if any non-final transfer has cs_change = true.

This is not the case, spi_message_max_size() is a limit on the size of a
spi_message.

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Re: [PATCH v3 net-next 1/2] net: dsa: sja1105: send multiple spi_messages instead of using cs_change

[Vladimir Oltean]

On Mon, May 24, 2021 at 09:35:29AM +0100, Mark Brown wrote:
> On Fri, May 21, 2021 at 12:16:56AM +0300, Vladimir Oltean wrote:
> 
> > The fact of the matter is that spi_max_message_size() has an ambiguous
> > meaning if any non-final transfer has cs_change = true.
> 
> This is not the case, spi_message_max_size() is a limit on the size of a
> spi_message.

That is true, although it doesn't mean much, since in the presence of
cs_change, a spi_message has no correspondent in the physical world
(i.e. you can't look at a logic analyzer dump and say "this spi_message
was from this to this point"), and that is the problem really.
Describing the controller's inability to send more than N SPI words with
continuous chip select using spi_message_max_size() is what seems flawed
to me, but it's what we have, and what I've adapted to.

Re: [PATCH v3 net-next 1/2] net: dsa: sja1105: send multiple spi_messages instead of using cs_change

[Mark Brown]

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On Mon, May 24, 2021 at 04:02:12PM +0300, Vladimir Oltean wrote:
> On Mon, May 24, 2021 at 09:35:29AM +0100, Mark Brown wrote:

> > This is not the case, spi_message_max_size() is a limit on the size of a
> > spi_message.

> That is true, although it doesn't mean much, since in the presence of
> cs_change, a spi_message has no correspondent in the physical world
> (i.e. you can't look at a logic analyzer dump and say "this spi_message
> was from this to this point"), and that is the problem really.

It may affect how things are implemented by the driver, for example if
the driver can send a command stream to the hardware the limit might be
due to that command stream.  There is no need or expectation for drivers
to pattern match what the're being asked to do and parse out something
that should be a string of messages from the spi_message they get, it is
expected that client drivers should split things up naturally.

> Describing the controller's inability to send more than N SPI words with
> continuous chip select using spi_message_max_size() is what seems flawed
> to me, but it's what we have, and what I've adapted to.

I can't entirely parse that but the limit here isn't to do with how long
chip select is asserted for.

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[PATCH v3 net-next 2/2] net: dsa: sja1105: adapt to a SPI controller with a limited max transfer size

[Vladimir Oltean]

From: Vladimir Oltean <vladimir.oltean@nxp.com>

The static config of the sja1105 switch is a long stream of bytes which
is programmed to the hardware in chunks (portions with the chip select
continuously asserted) of max 256 bytes each. Each chunk is a
spi_message composed of 2 spi_transfers: the buffer with the data and a
preceding buffer with the SPI access header.

Only that certain SPI controllers, such as the spi-sc18is602 I2C-to-SPI
bridge, cannot keep the chip select asserted for that long.
The spi_max_transfer_size() and spi_max_message_size() functions are how
the controller can impose its hardware limitations upon the SPI
peripheral driver.

For the sja1105 driver to work with these controllers, both buffers must
be smaller than the transfer limit, and their sum must be smaller than
the message limit.

Regression-tested on a switch connected to a controller with no
limitations (spi-fsl-dspi) as well as with one with caps for both
max_transfer_size and max_message_size (spi-sc18is602).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 drivers/net/dsa/sja1105/sja1105.h             |  1 +
 drivers/net/dsa/sja1105/sja1105_main.c        | 28 +++++++++++++++++++
 drivers/net/dsa/sja1105/sja1105_spi.c         | 16 +++++------
 .../net/dsa/sja1105/sja1105_static_config.h   |  2 ++
 4 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/drivers/net/dsa/sja1105/sja1105.h b/drivers/net/dsa/sja1105/sja1105.h
index f9e87fb33da0..7ec40c4b2d5a 100644
--- a/drivers/net/dsa/sja1105/sja1105.h
+++ b/drivers/net/dsa/sja1105/sja1105.h
@@ -209,6 +209,7 @@ struct sja1105_private {
 	unsigned long ucast_egress_floods;
 	unsigned long bcast_egress_floods;
 	const struct sja1105_info *info;
+	size_t max_xfer_len;
 	struct gpio_desc *reset_gpio;
 	struct spi_device *spidev;
 	struct dsa_switch *ds;
diff --git a/drivers/net/dsa/sja1105/sja1105_main.c b/drivers/net/dsa/sja1105/sja1105_main.c
index 1d2d34d40f84..585142caa5b0 100644
--- a/drivers/net/dsa/sja1105/sja1105_main.c
+++ b/drivers/net/dsa/sja1105/sja1105_main.c
@@ -3563,6 +3563,7 @@ static int sja1105_probe(struct spi_device *spi)
 	struct sja1105_tagger_data *tagger_data;
 	struct device *dev = &spi->dev;
 	struct sja1105_private *priv;
+	size_t max_xfer, max_msg;
 	struct dsa_switch *ds;
 	int rc, port;
 
@@ -3596,6 +3597,33 @@ static int sja1105_probe(struct spi_device *spi)
 		return rc;
 	}
 
+	/* In sja1105_xfer, we send spi_messages composed of two spi_transfers:
+	 * a small one for the message header and another one for the current
+	 * chunk of the packed buffer.
+	 * Check that the restrictions imposed by the SPI controller are
+	 * respected: the chunk buffer is smaller than the max transfer size,
+	 * and the total length of the chunk plus its message header is smaller
+	 * than the max message size.
+	 * We do that during probe time since the maximum transfer size is a
+	 * runtime invariant.
+	 */
+	max_xfer = spi_max_transfer_size(spi);
+	max_msg = spi_max_message_size(spi);
+
+	/* We need to send at least one 64-bit word of SPI payload per message
+	 * in order to be able to make useful progress.
+	 */
+	if (max_msg < SJA1105_SIZE_SPI_MSG_HEADER + 8) {
+		dev_err(dev, "SPI master cannot send large enough buffers, aborting\n");
+		return -EINVAL;
+	}
+
+	priv->max_xfer_len = SJA1105_SIZE_SPI_MSG_MAXLEN;
+	if (priv->max_xfer_len > max_xfer)
+		priv->max_xfer_len = max_xfer;
+	if (priv->max_xfer_len > max_msg - SJA1105_SIZE_SPI_MSG_HEADER)
+		priv->max_xfer_len = max_msg - SJA1105_SIZE_SPI_MSG_HEADER;
+
 	priv->info = of_device_get_match_data(dev);
 
 	/* Detect hardware device */
diff --git a/drivers/net/dsa/sja1105/sja1105_spi.c b/drivers/net/dsa/sja1105/sja1105_spi.c
index 8746e3f158a0..5a7b404bf3ce 100644
--- a/drivers/net/dsa/sja1105/sja1105_spi.c
+++ b/drivers/net/dsa/sja1105/sja1105_spi.c
@@ -8,8 +8,6 @@
 #include "sja1105.h"
 
 #define SJA1105_SIZE_RESET_CMD		4
-#define SJA1105_SIZE_SPI_MSG_HEADER	4
-#define SJA1105_SIZE_SPI_MSG_MAXLEN	(64 * 4)
 
 struct sja1105_chunk {
 	u8	*buf;
@@ -40,19 +38,19 @@ static int sja1105_xfer(const struct sja1105_private *priv,
 			size_t len, struct ptp_system_timestamp *ptp_sts)
 {
 	u8 hdr_buf[SJA1105_SIZE_SPI_MSG_HEADER] = {0};
-	struct sja1105_chunk chunk = {
-		.len = min_t(size_t, len, SJA1105_SIZE_SPI_MSG_MAXLEN),
-		.reg_addr = reg_addr,
-		.buf = buf,
-	};
 	struct spi_device *spi = priv->spidev;
 	struct spi_transfer xfers[2] = {0};
 	struct spi_transfer *chunk_xfer;
 	struct spi_transfer *hdr_xfer;
+	struct sja1105_chunk chunk;
 	int num_chunks;
 	int rc, i = 0;
 
-	num_chunks = DIV_ROUND_UP(len, SJA1105_SIZE_SPI_MSG_MAXLEN);
+	num_chunks = DIV_ROUND_UP(len, priv->max_xfer_len);
+
+	chunk.reg_addr = reg_addr;
+	chunk.buf = buf;
+	chunk.len = min_t(size_t, len, priv->max_xfer_len);
 
 	hdr_xfer = &xfers[0];
 	chunk_xfer = &xfers[1];
@@ -104,7 +102,7 @@ static int sja1105_xfer(const struct sja1105_private *priv,
 		chunk.buf += chunk.len;
 		chunk.reg_addr += chunk.len / 4;
 		chunk.len = min_t(size_t, (ptrdiff_t)(buf + len - chunk.buf),
-				  SJA1105_SIZE_SPI_MSG_MAXLEN);
+				  priv->max_xfer_len);
 
 		rc = spi_sync_transfer(spi, xfers, 2);
 		if (rc < 0) {
diff --git a/drivers/net/dsa/sja1105/sja1105_static_config.h b/drivers/net/dsa/sja1105/sja1105_static_config.h
index bc7606899289..779eb6840f05 100644
--- a/drivers/net/dsa/sja1105/sja1105_static_config.h
+++ b/drivers/net/dsa/sja1105/sja1105_static_config.h
@@ -9,6 +9,8 @@
 #include <linux/types.h>
 #include <asm/types.h>
 
+#define SJA1105_SIZE_SPI_MSG_HEADER			4
+#define SJA1105_SIZE_SPI_MSG_MAXLEN			(64 * 4)
 #define SJA1105_SIZE_DEVICE_ID				4
 #define SJA1105_SIZE_TABLE_HEADER			12
 #define SJA1105_SIZE_SCHEDULE_ENTRY			8
-- 
2.25.1

Re: [PATCH v3 net-next 0/2] Adapt the sja1105 DSA driver to the SPI controller's transfer limits

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (refs/heads/master):

On Fri, 21 May 2021 00:16:55 +0300 you wrote:
> From: Vladimir Oltean <vladimir.oltean@nxp.com>
> 
> This series changes the SPI transfer procedure in sja1105 to take into
> consideration the buffer size limitations that the SPI controller driver
> might have.
> 
> Changes in v3:
> - Avoid a signed vs unsigned issue in the interpretation of SIZE_MAX.
> - Move the max transfer length checks to probe time, since nothing will
>   change dynamically.
> 
> [...]

Here is the summary with links:
  - [v3,net-next,1/2] net: dsa: sja1105: send multiple spi_messages instead of using cs_change
    https://git.kernel.org/netdev/net-next/c/ca021f0dd851
  - [v3,net-next,2/2] net: dsa: sja1105: adapt to a SPI controller with a limited max transfer size
    https://git.kernel.org/netdev/net-next/c/718bad0e4da9

You are awesome, thank you!
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