On Tue, Mar 08, 2011 at 08:58:20PM -0800, Saravana Kannan wrote: > On 03/03/2011 02:24 AM, Russell King - ARM Linux wrote: >> On Wed, Mar 02, 2011 at 11:49:47PM -0800, Saravana Kannan wrote: >>>> I think you misunderstand what's going on. IO accesses are always ordered >>>> with respect to themselves. The barriers are there to ensure ordering >>>> between DMA coherent memory (normal non-cached memory) and IO accesses >>>> (device). >>> >>> Unfortunately this is not correct. The ARM spec doesn't guarantee that >>> all IO accesses should be ordered with respect to themselves. It only >>> requires that the ordering should be guaranteed at least within a 1KB >>> region. >>> >>> You can find this info in ARMv7 ARM spec[1] named >>> "DDI0406B_arm_architecture_reference_manual_errata_markup_8_0.pdf", on >>> page A3-45. There is a para that goes: >>> >>> "Accesses must arrive at any particular memory-mapped peripheral or >>> block of memory in program order, that is, A1 must arrive before A2. >>> There are no ordering restrictions about when accesses arrive at >>> different peripherals or blocks of memory, provided that the accesses >>> follow the general ordering rules given in this section." >> >> That is news to me. My DDI0406B does not have this paragraph, so it's >> something that ARM has sprung upon us without telling *anyone* about it. >> It's not unreasonable or even unexpected. That is exactly the same >> condition which applies on buses like PCI due to write posting on bridges >> downstream of the CPU, and issuing memory barriers will not help with >> that. > > While the PCI stuff is true, as you say, it's not related to mb()s. The > mb()s matter to the point of getting the writes to the intended > "devices addresses" in the program order. What happens after that is a > separate issue. > > So, going back to the discussion of mb()s and readl/writel (and > variations), I think we should no longer state the all IO accesses are > ordered wrt each other. Are we in agreement on this? No, because you clearly haven't understood the point I made. I still believe you are wrong on this point. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
