spi: pxa2xx: Set controller->max_transfer_size in dma mode

In DMA mode we have a maximum transfer size, past that the driver
falls back to PIO (see the check at the top of pxa2xx_spi_transfer_one).
Falling back to PIO for big transfers defeats the point of a dma engine,
hence set the max transfer size to inform spi clients that they need
to do something smarter.

This was uncovered by the drm_mipi_dbi spi panel code, which does
large spi transfers, but stopped splitting them after:

commit e143364
Author: Noralf Trønnes <[email protected]>
Date:   Fri Jul 19 17:59:10 2019 +0200

    drm/tinydrm: Remove tinydrm_spi_max_transfer_size()

After this commit the code relied on the spi core to split transfers
into max dma-able blocks, which also papered over the PIO fallback issue.

Fix this by setting the overall max transfer size to the DMA limit,
but only when the controller runs in DMA mode.

Fixes: e143364 ("drm/tinydrm: Remove tinydrm_spi_max_transfer_size()")
Cc: Sam Ravnborg <[email protected]>
Cc: Noralf Trønnes <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Reported-and-tested-by: Andy Shevchenko <[email protected]>
Cc: Daniel Mack <[email protected]>
Cc: Haojian Zhuang <[email protected]>
Cc: Robert Jarzmik <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

drivers/spi/spi-pxa2xx.c

@@ -1602,6 +1602,11 @@ static int pxa2xx_spi_fw_translate_cs(struct spi_controller *controller,
 	return cs;
 }
 
+static size_t pxa2xx_spi_max_dma_transfer_size(struct spi_device *spi)
+{
+	return MAX_DMA_LEN;
+}
+
 static int pxa2xx_spi_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
@@ -1707,6 +1712,8 @@ static int pxa2xx_spi_probe(struct platform_device *pdev)
 		} else {
 			controller->can_dma = pxa2xx_spi_can_dma;
 			controller->max_dma_len = MAX_DMA_LEN;
+			controller->max_transfer_size =
+				pxa2xx_spi_max_dma_transfer_size;
 		}
 	}