spi: spi-mem: Add a new controller capability

There are spi devices with multiple frequency limitations depending on
the invoked command. We probably do not want to afford running at the
lowest supported frequency all the time, so if we want to get the most
of our hardware, we need to allow per-operation frequency limitations.

Among all the SPI memory controllers, I believe all are capable of
changing the spi frequency on the fly. Some of the drivers do not make
any frequency setup though. And some others will derive a per chip
prescaler value which will be used forever.

Actually changing the frequency on the fly is something new in Linux, so
we need to carefully flag the drivers which do and do not support it. A
controller capability is created for that, and the presence for this
capability will always be checked before accepting such pattern.

Signed-off-by: Miquel Raynal <[email protected]>
Reviewed-by: Tudor Ambarus <[email protected]>
Link: https://patch.msgid.link/20241224-winbond-6-11-rc1-quad-support-v2-2-ad218dbc406f@bootlin.com
Signed-off-by: Mark Brown <[email protected]>

Author: miquelraynal

drivers/spi/spi-mem.c

@@ -191,6 +191,12 @@ bool spi_mem_default_supports_op(struct spi_mem *mem,
 	    op->max_freq < mem->spi->controller->min_speed_hz)
 		return false;
 
+	if (op->max_freq &&
+	    op->max_freq < mem->spi->max_speed_hz) {
+		if (!spi_mem_controller_is_capable(ctlr, per_op_freq))
+			return false;
+	}
+
 	return spi_mem_check_buswidth(mem, op);
 }
 EXPORT_SYMBOL_GPL(spi_mem_default_supports_op);

include/linux/spi/spi-mem.h

@@ -311,11 +311,13 @@ struct spi_controller_mem_ops {
  * @ecc: Supports operations with error correction
  * @swap16: Supports swapping bytes on a 16 bit boundary when configured in
  *	    Octal DTR
+ * @per_op_freq: Supports per operation frequency switching
  */
 struct spi_controller_mem_caps {
 	bool dtr;
 	bool ecc;
 	bool swap16;
+	bool per_op_freq;
 };
 
 #define spi_mem_controller_is_capable(ctlr, cap)	\