Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device

On Wed, 24 Feb 2010, Rick L. Vinyard, Jr. wrote:

quoted

quoted

+static int picolcd_raw_event(struct hid_device *hdev,
+               struct hid_report *report, u8 *raw_data, int size)
+{
+       struct picolcd_data *data = hid_get_drvdata(hdev);
+       char hexdata[25];
+       int i;
+
+       if (data = NULL)
+               return 1;
+
+       for (i = 0; i < sizeof(hexdata) / 2; i++)
+               sprintf(hexdata+2*i, "%02hhx", raw_data[i]);
+       if (size >= sizeof(hexdata)/2) {
+               hexdata[sizeof(hexdata)-4] = '.';
+               hexdata[sizeof(hexdata)-3] = '.';
+               hexdata[sizeof(hexdata)-2] = '.';
+               hexdata[sizeof(hexdata)-1] = '\0';
+       } else
+               hexdata[size*2] = '\0';
+
+       switch (report->id) {
+       case REPORT_KEYPAD:
+               if (size = 3 && raw_data[0] = 0x11 &&
data->input_keys) {
+                       return picolcd_raw_keypad(hdev, report,
raw_data+1, size-1);
+               } else {
+                       dbg_hid(PICOLCD_NAME " unsupported key event (%d
bytes): 0x%s\n", size, hexdata);
+                       break;
+               }
+               break;
+       case REPORT_VERSION:
+               if (size = 3)
+                       dev_info(&hdev->dev, "Firmware version is
%hd.%hd\n", raw_data[1], raw_data[2]);
+
+               spin_lock(&data->lock);

If I recall correctly raw_event is called in interrupt. 

Yes, that is correct.

The issue, as I understand it is that non-interrupt code may obtain the 
lock and then the interrupt code is executed... hence the deadlock and 
the need to use spin_lock_irqsave() and spin_unlock_irqrestore().

Exactly. All the spinlocks that are aquired in interrupt code-paths must 
be acquired with _irqsave()/_irqrestore() from the non-interrupt code to 
prevent exactly this kind of deadlock.

The key difference is the replacement of spin_lock() with spin_trylock()
such that if the non-interrupt code has already obtained the lock, the
interrupt will not deadlock but instead take the else path and schedule a
framebuffer update at the next interval.

Why is _irqsave() and/or deferred work not enough? The aproach with 
_trylock() seems to be overly complicated for no good reason (I personally 
become very suspicious every time I see code that is using _trylock()).

[ by the way, Rick, are you planning to resubmit the G13 driver with 
incorporated feedback from the last review round? ]

Thanks,

-- 
Jiri Kosina
SUSE Labs, Novell Inc.