linux-kernel - Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions

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Date:   Fri, 17 May 2019 13:08:25 -0500
From:   Alex Elder <elder@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     David Miller <davem@...emloft.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        syadagir@...eaurora.org, mjavid@...eaurora.org,
        evgreen@...omium.org, benchan@...gle.com, ejcaruso@...gle.com,
        abhishek.esse@...il.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions

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?

					-Alex