Date:   Tue, 4 Apr 2017 18:18:56 +0100
From:   Alexey Klimov <alexey.klimov@....com>
To:     George Cherian <gcherian@...iumnetworks.com>
CC:     George Cherian <george.cherian@...ium.com>,
        <linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <devel@...ica.org>, <ashwin.chaugule@...aro.org>,
        <rjw@...ysocki.net>, <lenb@...nel.org>, <jassisinghbrar@...il.com>,
        <robert.moore@...el.com>, <lv.zheng@...el.com>,
        <pprakash@...eaurora.org>
Subject: Re: [PATCH 2/2] ACPI / CPPC: Make cppc acpi driver aware of pcc
 subspace ids

On Tue, 4 Apr 2017 16:21:20 +0530 George Cherian <gcherian@...iumnetworks.com> wrote:

>
> Hi Alexey,
>
> On 04/03/2017 11:07 PM, Alexey Klimov wrote:
> > (adding Prashanth to c/c)
> >
> > Hi George,
> >
> > On Fri, Mar 31, 2017 at 06:24:02AM +0000, George Cherian wrote:
> >> Based on Section 14.1 of ACPI specification, it is possible to
> >> have a maximum of 256 PCC subspace ids. Add support of multiple
> >> PCC subspace id instead of using a single global pcc_data
> >> structure.
> >>
> >> While at that fix the time_delta check in send_pcc_cmd() so that
> >> last_mpar_reset and mpar_count is initialized properly. Also
> >> maintain a global total_mpar_count which is a sum of per subspace
> >> id mpar value.
> >
> > Could you please provide clarification on why sum of
> > total_mpar_count is required? Do you assume that there always will
> > be only one single firmware CPU that handles PCC commands on
> > another side?
>
> Yes you are right the total_mpar_count  should be removed and should
> be handled per subspace id. Moreover the current logic of not sending
> the command to PCC and returning with -EIO is also flawed. It should
> actually have a retry mechanism instead of returning -EIO even
> without submitting the request to the channel.

That sounds interesting.
How many times should the code try to resend before giving up (let's
say that timing constraints allow the caller to resend command)?

Regarding error codes, the code can differentiate between timeout,
platform error, timing constraints (-EBUSY?), maybe some other errors.
In some cases and since mailbox framework can't resend pcc commands on
itself then it's the client responsibility to re-queue a command.


> > Theoretically different PCC channels can be connected to different
> > platform CPUs on other end (imagine NUMA systems in case of CPPC)
> > so it's not clear why transport layer of PCC should use that global
> > count. Also, ACPI spec 6.1 (page 701) in in description of MPAR
> > states "The maximum number of periodic requests that the subspace
> > channel can support".
> >
> >
> >
> >> Signed-off-by: George Cherian <george.cherian@...ium.com>
> >> ---
> >>   drivers/acpi/cppc_acpi.c | 189
> >> ++++++++++++++++++++++++++--------------------- 1 file changed,
> >> 105 insertions(+), 84 deletions(-)
> >>
> >> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> >> index 3ca0729..7ba05ac 100644
> >> --- a/drivers/acpi/cppc_acpi.c
> >> +++ b/drivers/acpi/cppc_acpi.c
> >> @@ -77,12 +77,16 @@ struct cppc_pcc_data {
> >>    wait_queue_head_t pcc_write_wait_q;
> >>   };
> >>
> >> -/* Structure to represent the single PCC channel */
> >> -static struct cppc_pcc_data pcc_data = {
> >> -  .pcc_subspace_idx = -1,
> >> -  .platform_owns_pcc = true,
> >> -};
> >> +/* Array  to represent the PCC channel per subspace id */
> >> +static struct cppc_pcc_data pcc_data[MAX_PCC_SUBSPACES];
> >> +/*
> >> + * It is quiet possible that multiple CPU's can share
> >> + * same subspace ids. The cpu_pcc_subspace_idx maintains
> >> + * the cpu to pcc subspace id map.
> >> + */
> >> +static DEFINE_PER_CPU(int, cpu_pcc_subspace_idx);
> >>
> >> +static int total_mpar_count;
> >>   /*
> >>    * The cpc_desc structure contains the ACPI register details
> >>    * as described in the per CPU _CPC tables. The details
> >> @@ -93,7 +97,8 @@ static struct cppc_pcc_data pcc_data = {
> >>   static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
> >>
> >>   /* pcc mapped address + header size + offset within PCC subspace
> >> */ -#define GET_PCC_VADDR(offs) (pcc_data.pcc_comm_addr + 0x8 +
> >> (offs)) +#define GET_PCC_VADDR(offs, pcc_ss_id)
> >> (pcc_data[pcc_ss_id].pcc_comm_addr + \
> >> +                                          0x8 + (offs))
> >>
> >>   /* Check if a CPC regsiter is in PCC */
> >>   #define CPC_IN_PCC(cpc) ((cpc)->type == ACPI_TYPE_BUFFER
> >> &&         \ @@ -183,13 +188,17 @@ static struct kobj_type
> >> cppc_ktype = { .default_attrs = cppc_attrs,
> >>   };
> >>
> >> -static int check_pcc_chan(bool chk_err_bit)
> >> +static int check_pcc_chan(int cpunum, bool chk_err_bit)
> >>   {
> >>    int ret = -EIO, status = 0;
> >> -  struct acpi_pcct_shared_memory __iomem *generic_comm_base
> >> = pcc_data.pcc_comm_addr;
> >> -  ktime_t next_deadline = ktime_add(ktime_get(),
> >> pcc_data.deadline); -
> >> -  if (!pcc_data.platform_owns_pcc)
> >> +  int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpunum);
> >> +  struct cppc_pcc_data *pcc_ss_data = &pcc_data[pcc_ss_id];
> >> +  struct acpi_pcct_shared_memory __iomem *generic_comm_base
> >> =
> >> +          pcc_ss_data->pcc_comm_addr;
> >> +  ktime_t next_deadline = ktime_add(ktime_get(),
> >> +                                    pcc_ss_data->deadline);
> >> +
> >> +  if (!pcc_ss_data->platform_owns_pcc)
> >>            return 0;
> >>
> >>    /* Retry in case the remote processor was too slow to
> >> catch up. */ @@ -214,7 +223,7 @@ static int check_pcc_chan(bool
> >> chk_err_bit) }
> >>
> >>    if (likely(!ret))
> >> -          pcc_data.platform_owns_pcc = false;
> >> +          pcc_ss_data->platform_owns_pcc = false;
> >>    else
> >>            pr_err("PCC check channel failed. Status=%x\n",
> >> status);
> >>
> >> @@ -225,11 +234,13 @@ static int check_pcc_chan(bool chk_err_bit)
> >>    * This function transfers the ownership of the PCC to the
> >> platform
> >>    * So it must be called while holding write_lock(pcc_lock)
> >>    */
> >> -static int send_pcc_cmd(u16 cmd)
> >> +static int send_pcc_cmd(int cpunum, u16 cmd)
> >
> >
> > I don't like the direction of where it's going.
> >
> > To send commands through PCC channel you don't need to know CPU
> > number. Ideally, send_pcc_cmd() shouldn't care a lot about software
> > entity that uses it (CPPC, RASF, MPST, etc) and passing some CPU
> > number to this function you bind it to CPPC interfaces while it
> > shouldn't depend on it. Maybe you can pass subspace it here instead.
> Actually if you look inside send_pcc_cmd it does a mapping of cpu to
> subspace id.  To avoid confusion it is better to pass the subspace id
> to this function than the cpu number.

That is what I tried to suggest in my comments.


> > BTW, is it possible to make separate mailbox PCC client and move it
> > out from CPPC code?
>
> Why do you think it is necessary? Currently CPPC driver itself is the
> client for mailbox/pcc driver.

There is no reason to copy-paste client code.

I didn't find if it's allowed by ACPI spec for few entities as MPST and
CPPC to use the same PCC channel for example but if yes, then separate
client code should care about queuing, locking, re-trying.



Thanks,
Alexey.

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