Re: Recommended functions for accessing internal registers
I'm on an embedded system, so every resource counts. One of the biggest impacts is when writing to a communication/memory access FIFO or reading/writing configurations. In these cases, I'd just need to make sure that there's no I/O reordering and/or subsequent r/w are not optimized away, I believe. Should I switch to the deprecated volatile attribute? Thank you. Cheers, Matteo Il 02/12/2009 21.57, Benjamin Herrenschmidt ha scritto: On Tue, 2009-12-01 at 17:44 +0100, Fortini Matteo wrote: I see that throughout the kernel source, internal PPC registers are accessed through [in|out]_be[32|16|8]() functions. However, they are translated into 3 inline assembly instructions, one of which is an isync, which has a huge performance hit. I tried using readl_be() which seems to be the right function according to the Documentation/ dir, but it is translated directly to in_be32(), so no luck. Is it really necessary to use all those instructions? I know I could use a (volatile u32 *) variable to avoid subsequent read/writes to be optimized out, but it seems to be a deprecated use. There are good reasons why the accessors contain those barriers. What are you doing that would be performance critical enough for those to be a problem ? Cheers, Ben. ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: Recommended functions for accessing internal registers
On Thu, 2009-12-03 at 12:16 +0100, Fortini Matteo wrote: I'm on an embedded system, so every resource counts. One of the biggest impacts is when writing to a communication/memory access FIFO or reading/writing configurations. In these cases, I'd just need to make sure that there's no I/O reordering and/or subsequent r/w are not optimized away, I believe. Should I switch to the deprecated volatile attribute? If it's a single register fifo, you can use the _outs/_ins functions or the iomap.h variants which are cleaner. If it's a linear region, look at memcpy_to_io... You can always go directly poking at it but you need appropriate memory barriers in and around your accesses. The reason there's a twi/isync pair inside in_* is to ensure that the read is actually performed immediately. Without this, it could be delayed until the CPU decides to consume the data which could cause timing issues when a read is followed by a delay for example. There's a few other reasons why it's a good idea to do so. Cheers, Ben. Thank you. Cheers, Matteo Il 02/12/2009 21.57, Benjamin Herrenschmidt ha scritto: On Tue, 2009-12-01 at 17:44 +0100, Fortini Matteo wrote: I see that throughout the kernel source, internal PPC registers are accessed through [in|out]_be[32|16|8]() functions. However, they are translated into 3 inline assembly instructions, one of which is an isync, which has a huge performance hit. I tried using readl_be() which seems to be the right function according to the Documentation/ dir, but it is translated directly to in_be32(), so no luck. Is it really necessary to use all those instructions? I know I could use a (volatile u32 *) variable to avoid subsequent read/writes to be optimized out, but it seems to be a deprecated use. There are good reasons why the accessors contain those barriers. What are you doing that would be performance critical enough for those to be a problem ? Cheers, Ben. ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: Recommended functions for accessing internal registers
On Tue, 2009-12-01 at 17:44 +0100, Fortini Matteo wrote: I see that throughout the kernel source, internal PPC registers are accessed through [in|out]_be[32|16|8]() functions. However, they are translated into 3 inline assembly instructions, one of which is an isync, which has a huge performance hit. I tried using readl_be() which seems to be the right function according to the Documentation/ dir, but it is translated directly to in_be32(), so no luck. Is it really necessary to use all those instructions? I know I could use a (volatile u32 *) variable to avoid subsequent read/writes to be optimized out, but it seems to be a deprecated use. There are good reasons why the accessors contain those barriers. What are you doing that would be performance critical enough for those to be a problem ? Cheers, Ben. ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Recommended functions for accessing internal registers
I see that throughout the kernel source, internal PPC registers are accessed through [in|out]_be[32|16|8]() functions. However, they are translated into 3 inline assembly instructions, one of which is an isync, which has a huge performance hit. I tried using readl_be() which seems to be the right function according to the Documentation/ dir, but it is translated directly to in_be32(), so no luck. Is it really necessary to use all those instructions? I know I could use a (volatile u32 *) variable to avoid subsequent read/writes to be optimized out, but it seems to be a deprecated use. Thank you in advance, Matteo ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
