[PATCH v1] usb: xhci: allow imod-interval to be configurable

On 11/28/2017 2:35 PM, Greg Kroah-Hartman wrote:

On Tue, Nov 28, 2017 at 12:11:46PM -0500, Adam Wallis wrote:

quoted

The xHCI driver currently has the IMOD set to 160, which
translates to an IMOD interval of 40,000ns (160 * 250)ns

[..]

quoted

--- a/drivers/usb/host/xhci-plat.c
+++ b/drivers/usb/host/xhci-plat.c

@@ -23,6 +23,7 @@
 #include "xhci-plat.h"
 #include "xhci-mvebu.h"
 #include "xhci-rcar.h"
+#include "xhci-mtk.h"
 
 static struct hc_driver __read_mostly xhci_plat_hc_driver;
 

@@ -269,6 +270,20 @@ static int xhci_plat_probe(struct platform_device *pdev)
 	if (device_property_read_bool(&pdev->dev, "quirk-broken-port-ped"))
 		xhci->quirks |= XHCI_BROKEN_PORT_PED;
 
+	/* imod interval in nanoseconds */
+	if (device_property_read_u32(sysdev,
+		"imod-interval", &xhci->imod_interval))
+		xhci->imod_interval = 40000;

So no matter what value you read, you set it to 40000?  Or am I reading
this code incorrectly?

I think you may be reading the code incorrectly. device_property_read_u32()
returns 0 when the property is found and valid...and stored into
xhci->imod_interval. When 0 is returned in this case, the default value of
40,000 is skipped over.

If the property is not found, a number of different errors could be returned,
but any of them will cause the default value to be used.

There's a good reason putting function calls inside if() is considered a
bad coding style :)

I do not disagree with you, however, I was trying to maintain style consistency
with the device property reads with the xhci_plat_probe function.

If I break that consistency, a couple of ways I might write this cleaner

1) set xhci->imod_interval to 40,000 before the call to
device_property_read_u32. If the property exists in a firmware node, it will
update the imod_interval value...if it does not exist, it will not update this
value and the default will be used. In this case, I would not even check the
return value. This method is used quite a bit in the kernel.

2) use a if (device_property_present()) and check to see if that property is
even present. If so, call device_property_read_u32() without check return value.
This has the downside of still using a function call within the if() which you
have already indicated is not your preference.

3) add a retval and test off of this instead of using the function directly in
the if()

thanks,

greg k-h

Thanks for taking time to review my patch. I really have no strong preference on
how call device_property_read_u32() is tested and I am open to any suggestions.
Adam


-- 
Adam Wallis
Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.