lkml.org 
[lkml]   [2019]   [May]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 09/18] soc: qcom: ipa: GSI transactions
    From
    Date
    On 5/17/19 1:33 PM, Arnd Bergmann wrote:
    > On Fri, May 17, 2019 at 8:08 PM Alex Elder <elder@linaro.org> wrote:
    >>
    >> On 5/15/19 2:34 AM, Arnd Bergmann wrote:
    >>>> +static void gsi_trans_tre_fill(struct gsi_tre *dest_tre, dma_addr_t addr,
    >>>> + u32 len, bool last_tre, bool bei,
    >>>> + enum ipa_cmd_opcode opcode)
    >>>> +{
    >>>> + struct gsi_tre tre;
    >>>> +
    >>>> + tre.addr = cpu_to_le64(addr);
    >>>> + tre.len_opcode = gsi_tre_len_opcode(opcode, len);
    >>>> + tre.reserved = 0;
    >>>> + tre.flags = gsi_tre_flags(last_tre, bei, opcode);
    >>>> +
    >>>> + *dest_tre = tre; /* Write TRE as a single (16-byte) unit */
    >>>> +}
    >>> Have you checked that the atomic write is actually what happens here,
    >>> but looking at the compiler output? You might need to add a 'volatile'
    >>> qualifier to the dest_tre argument so the temporary structure doesn't
    >>> get optimized away here.
    >>
    >> Currently, the assignment *does* become a "stp" instruction.
    >> But I don't know that we can *force* the compiler to write it
    >> as a pair of registers, so I'll soften the comment with
    >> "Attempt to write" or something similar.
    >>
    >> To my knowledge, adding a volatile qualifier only prevents the
    >> compiler from performing funny optimizations, but that has no
    >> effect on whether the 128-bit assignment is made as a single
    >> unit. Do you know otherwise?
    >
    > I don't think it you can force the 128-bit assignment to be
    > atomic, but marking 'dest_tre' should serve to prevent a
    > specific optimization that replaces the function with
    >
    > dest_tre->addr = ...
    > dest_tre->len_opcode = ...
    > dest_tre->reserved = ...
    > dest_tre->flags = ...
    >
    > which it might find more efficient than the stp and is equivalent
    > when the pointer is not marked volatile. We also have the WRITE_ONCE()
    > macro that can help prevent this, but it does not work reliably beyond
    > 64 bit assignments.

    OK, I'll mark it volatile to avoid that potential result.
    Thanks.

    -Alex

    >
    > Arnd
    >

    \
     
     \ /
      Last update: 2019-05-17 20:44    [W:2.359 / U:0.484 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site