David Hendricks has uploaded this change for review.

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linux_spi: Dynamically detect max buffer size

The max buffer size of the linux kernel's SPI device is a compile-time parameter.
Read it on initialization and use it to inform the max transfer size. We allow
up to 5 bytes for the SPI command. On master, SPI commands are always 4 bytes,
but 4ba support will raise that to 5. Choosing 5 makes sure this will keep
working when 4ba support is merged.

Change-Id: Ic541e548ced8488f074d388f1c92174cad123064
Signed-off-by: Keno Fischer <keno@juliacomputing.com>
---
M linux_spi.c
1 file changed, 18 insertions(+), 2 deletions(-)

git pull ssh://review.coreboot.org:29418/flashrom refs/changes/44/22344/1

diff --git a/linux_spi.c b/linux_spi.c
index e51fbc4..f7d9434 100644
--- a/linux_spi.c
+++ b/linux_spi.c
@@ -42,6 +42,7 @@
  */
 
 static int fd = -1;
+static size_t max_kernel_buf_size = 4096;
 
 static int linux_spi_shutdown(void *data);
 static int linux_spi_send_command(struct flashctx *flash, unsigned int writecnt,
@@ -127,6 +128,19 @@
 		return 1;
 	}
 
+	/* Try to read the kernel's max bufsize */
+	char buf[10];
+	memset(buf, 0, sizeof(buf));
+	int param_fd = open("/sys/module/spidev/parameters/bufsiz", O_RDONLY);
+	if (param_fd == -1 || read(param_fd, &buf, sizeof(buf) - 1) == -1) {
+		msg_pwarn("%s: failed to retrieve kernel buffer size. Attempting to use default.\n", __func__);
+	} else {
+		max_kernel_buf_size = atoi(buf);
+	}
+
+	if (param_fd != -1)
+		close(param_fd);
+
 	register_spi_master(&spi_master_linux);
 
 	return 0;
@@ -182,14 +196,16 @@
 static int linux_spi_read(struct flashctx *flash, uint8_t *buf,
 			  unsigned int start, unsigned int len)
 {
+	/* The size of the command and the buffer may add up to no more than max_kernel_buf_size.
+	   At the moment the command size is at most 5 bytes, so allow the rest for the buffer. */
 	return spi_read_chunked(flash, buf, start, len,
-				(unsigned int)getpagesize());
+				max_kernel_buf_size - 5);
 }
 
 static int linux_spi_write_256(struct flashctx *flash, const uint8_t *buf, unsigned int start, unsigned int len)
 {
 	return spi_write_chunked(flash, buf, start, len,
-				((unsigned int)getpagesize()) - 4);
+				 max_kernel_buf_size - 5);
 }
 
 #endif // CONFIG_LINUX_SPI == 1

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