tegra210-adma: fix 32-bit x86 build

The Tegra210 Audio DMA controller driver did a plain divide:

	page_no = (res_page->start - res_base->start) / cdata->ch_base_offset;

which causes problems on 32-bit x86 configurations that have 64-bit
resource sizes:

  x86_64-linux-ld: drivers/dma/tegra210-adma.o: in function `tegra_adma_probe':
  tegra210-adma.c:(.text+0x1322): undefined reference to `__udivdi3'

because gcc doesn't generate the trivial code for a 64-by-32 divide,
turning it into a function call to do a full 64-by-64 divide.  And the
kernel intentionally doesn't provide that helper function, because 99%
of the time all you want is the narrower version.

Of course, tegra210 is a 64-bit architecture and the 32-bit x86 build is
purely for build testing, so this really is just about build coverage
failure.

But build coverage is good.

Side note: div_u64() would be suboptimal if you actually have a 32-bit
resource_t, so our "helper" for divides are admittedly making it harder
than it should be to generate good code for all the possible cases.

At some point, I'll consider 32-bit x86 so entirely legacy that I can't
find it in myself to care any more, and we'll just add the __udivdi3
library function.

But for now, the right thing to do is to use "div_u64()" to show that
you know that you are doing the simpler divide with a 32-bit number.
And the build error enforces that.

While fixing the build issue, also check for division-by-zero, and for
overflow.  Which hopefully cannot happen on real production hardware,
but the value of 'ch_base_offset' can definitely be zero in other
places.

Reported-by: Guenter Roeck <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/dma/tegra210-adma.c

@@ -887,7 +887,7 @@ static int tegra_adma_probe(struct platform_device *pdev)
 	const struct tegra_adma_chip_data *cdata;
 	struct tegra_adma *tdma;
 	struct resource *res_page, *res_base;
-	int ret, i, page_no;
+	int ret, i;
 
 	cdata = of_device_get_match_data(&pdev->dev);
 	if (!cdata) {
@@ -914,9 +914,20 @@ static int tegra_adma_probe(struct platform_device *pdev)
 
 		res_base = platform_get_resource_byname(pdev, IORESOURCE_MEM, "global");
 		if (res_base) {
-			page_no = (res_page->start - res_base->start) / cdata->ch_base_offset;
-			if (page_no <= 0)
+			resource_size_t page_offset, page_no;
+			unsigned int ch_base_offset;
+
+			if (res_page->start < res_base->start)
+				return -EINVAL;
+			page_offset = res_page->start - res_base->start;
+			ch_base_offset = cdata->ch_base_offset;
+			if (!ch_base_offset)
 				return -EINVAL;
+
+			page_no = div_u64(page_offset, ch_base_offset);
+			if (!page_no || page_no > INT_MAX)
+				return -EINVAL;
+
 			tdma->ch_page_no = page_no - 1;
 			tdma->base_addr = devm_ioremap_resource(&pdev->dev, res_base);
 			if (IS_ERR(tdma->base_addr))