On Wed, 18 Mar 2015, Eric Anholt wrote: > Lee Jones <lee@xxxxxxxxxx> writes: > > > On Thu, 12 Mar 2015, Eric Anholt wrote: > > > >> From: Lubomir Rintel <lkundrak@xxxxx> > >> > >> Implement BCM2835 mailbox support as a device registered with the > >> general purpose mailbox framework. Implementation based on commits by > >> Lubomir Rintel [1], Suman Anna and Jassi Brar [2] on which to base the > >> implementation. > >> > >> [1] http://lists.infradead.org/pipermail/linux-rpi-kernel/2013-April/000528.html > >> [2] http://lists.infradead.org/pipermail/linux-rpi-kernel/2013-May/000546.html > >> > >> Signed-off-by: Lubomir Rintel <lkundrak@xxxxx> > >> Signed-off-by: Craig McGeachie <slapdau@xxxxxxxxxxxx> > >> Signed-off-by: Suman Anna <s-anna@xxxxxx> > >> Signed-off-by: Jassi Brar <jassisinghbrar@xxxxxxxxx> > >> Signed-off-by: Eric Anholt <eric@xxxxxxxxxx> > >> Cc: Jassi Brar <jassisinghbrar@xxxxxxxxx> > >> Acked-by: Lee Jones <lee.jones@xxxxxxxxxx> > >> --- > >> > >> > >> v2: Squashed Craig's work for review, carried over to new version of > >> Mailbox framework (changes by Lubomir) > >> > >> v3: Fix multi-line comment style. Refer to the documentation by > >> filename. Only declare one MODULE_AUTHOR. Alphabetize includes. > >> Drop some excessive dev_dbg()s (changes by anholt). > >> > >> v4: Use the new bcm2835_peripheral_read_workaround(), drop the > > > > Can you explain to me why this is required (and don't just point me in > > the direction of the other patch ;) ). You appear to be using the > > non-relaxed variants of readl and writel, which already do memory > > barriers, so I'm a little perplexed as to how the problem can arise. > > Hmm. > > A shorter restatement of the architecture requirement would be, I think, > "Don't let there be two outstanding reads of different peripherals on > the AXI bus, or the CPU might mis-assign the read results. Use rmb() to > wait for the previous bus reads when you need to prevent this" > > arch/arm/include/asm/io.h's readl() does __iormb() after each > __raw_readl(). Imagine taking an interrupt for a new peripheral between > the driver's __raw_readl() and the following __iormb(): Now you've got > two __raw_readl()s in between iormb()s and you can theoretically get > unordered reads. > > We could hope that the architecture IRQ handler would happen to do an > incidental rmb(), resolving the need to protect from interrupt handling > inside of device drivers. The interrupt controller's presence at > 0x7e00b200 sounds like it's an AXI peripheral, so it would need to be > ensuring ordering of reads. However, it's doing readl_relaxed()s. So > my rmb() at the start of my irq handler is silly -- if somebody got > interrupted between readl and rmb, we've already had a chance to get the > wrong result inside of the IRQ chip's status read. > > My new idea for handling this would be to: > > 1) Assume drivers don't exit with reads outstanding. This means they > don't do a readl_relaxed() from an AXI peripheral at the end of a path > without doing something with the result. > > 2) Make bcm2835_handle_irq() do this rmb() at the top, with the big > explanation, to avoid a race against the interrupted code device being > inside a readl() before the __iormb(). We don't worry about the 1-2 > readl_relaxed()s inside of bcm2835_handle_irq(), because their return > values get waited on before continuing on to calling the device driver, > so the device driver knows its IRQ handler is being entered with no AXI > reads outstanding. That's a fantastic explanation. Thanks for taking the time to write this out so diligently. Doing this at a sub-arch level sounds a little wrong to me. I don't think Broadcom are the only vendor who do not ensure correct read order, and writing <vendor>_peripheral_read_workarounds() all over the place sounds less than graceful. Granted, if there were a greater need and this can't be fixed another way we could knock off the <vendor>_ part and make the call generic, but is there no way we can deal with this at the architecture level? The whole point of doing readl_relaxed()s is that you can be assured that the architecture guarantee ordering. If that's not the case, then we need to be using readl()s in the IRQ handler instead. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
