drm/i915/gmbus: use generic poll_timeout*() instead of wait_for*()

Prefer generic poll helpers over i915 custom helpers.

The "two tier" wait_for_us() + wait_for() combination appeared without
much explanation in commit 4e6c2d58ba86 ("drm/i915: Take forcewake once
for the entire GMBUS transaction"). Try to mimic roughly the same with
the generic helpers.

wait_for_us() with 10 us or shorter timeouts ends up in
_wait_for_atomic(). Thus use poll_timeout_us_atomic() for the first try,
with the same 2 us timeout and no sleep.

For the fallback, the functional change is losing the exponentially
growing sleep of wait_for(), which used to be 10, 20, 40, ..., 640, and
1280 us. Use an arbitrary constant 500 us sleep instead. The timeout
remains at 50 ms.

Reviewed-by: Jouni Högander <[e-mail]>
Link: https://lore.kernel.org/r/564[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/i915/display/intel_gmbus.c

···
 #include <linux/export.h>
 #include <linux/i2c-algo-bit.h>
 #include <linux/i2c.h>
+#include <linux/iopoll.h>
 
 #include <drm/display/drm_hdcp_helper.h>
 
 #include "i915_drv.h"
 #include "i915_irq.h"
 #include "i915_reg.h"
-#include "i915_utils.h"
 #include "intel_de.h"
 #include "intel_display_regs.h"
 #include "intel_display_types.h"
···
 	intel_de_write_fw(display, GMBUS4(display), irq_en);
 
 	status |= GMBUS_SATOER;
-	ret = wait_for_us((gmbus2 = intel_de_read_fw(display, GMBUS2(display))) & status,
-			  2);
+
+	ret = poll_timeout_us_atomic(gmbus2 = intel_de_read_fw(display, GMBUS2(display)),
+				     gmbus2 & status,
+				     0, 2, false);
 	if (ret)
-		ret = wait_for((gmbus2 = intel_de_read_fw(display, GMBUS2(display))) & status,
-			       50);
+		ret = poll_timeout_us(gmbus2 = intel_de_read_fw(display, GMBUS2(display)),
+				      gmbus2 & status,
+				      500, 50 * 1000, false);
 
 	intel_de_write_fw(display, GMBUS4(display), 0);
 	remove_wait_queue(&display->gmbus.wait_queue, &wait);