Date:   Mon, 11 Jun 2018 13:47:15 +0000
From:   <Mario.Limonciello@...l.com>
To:     <dvhart@...radead.org>, <andy.shevchenko@...il.com>
Cc:     <stuart.w.hayes@...il.com>, <Douglas.Warzecha@...l.com>,
        <Jared.Dominguez@...l.com>, <linux-kernel@...r.kernel.org>,
        <platform-driver-x86@...r.kernel.org>
Subject: RE: [PATCH resend v2] dcdbas: Add support for WSMT ACPI table

> -----Original Message-----
> From: Darren Hart [mailto:dvhart@...radead.org]
> Sent: Friday, June 8, 2018 8:04 PM
> To: Andy Shevchenko
> Cc: Stuart Hayes; Warzecha, Douglas; Limonciello, Mario; Dominguez, Jared; Linux
> Kernel Mailing List; Platform Driver
> Subject: Re: [PATCH resend v2] dcdbas: Add support for WSMT ACPI table
> 
> On Thu, Jun 07, 2018 at 08:11:41PM +0300, Andy Shevchenko wrote:
> > On Thu, Jun 7, 2018 at 6:59 PM, Stuart Hayes <stuart.w.hayes@...il.com>
> wrote:
> > > If the WSMT ACPI table is present and indicates that a fixed communication
> > > buffer should be used, use the firmware-specified buffer instead of
> > > allocating a buffer in memory for communications between the dcdbas driver
> > > and firmare.
> >
> > >  config DCDBAS
> > >         tristate "Dell Systems Management Base Driver"
> > > -       depends on X86
> > > +       depends on X86 && ACPI
> 
> 
> >
> > Hmm... I'm not sure about this dependency.
> > So, the question is do all users actually need this? How did it work
> > previously? How has this been tested in case when command line has
> > "acpi=off" (yes, this one just for sake of test, I don't believe it's
> > a real use case)?
> 
> Yeah... after the 4.16 and 4.17 KConfig fumbling around the SMBIOS
> driver which intersected with this one.... this needs to be thoroughly
> covered, tested, and thought through. Linus was.... generous in the
> number of attempts it took us to get that right.
> 
> Did DCDBAS ever work on a system without ACPI?

Yes, I would expect it would have functioned on a system with kernel's
ACPI disabled.  However I also agree this isn't a real use case with a modern
Machine.

Perhaps the right thing to do is
#ifdef CONFIG_ACPI

For the WSMT relevant items in this patch?

> 
> >
> > >  #include <linux/string.h>
> > >  #include <linux/types.h>
> > >  #include <linux/mutex.h>
> > > +#include <linux/acpi.h>
> >
> > Please, try to keep an order as much as possible.
> > For example, in given context this line should be before string.h (I
> > didn't check the actual file, perhaps even upper).
> >
> > >  #include <asm/io.h>
> > >
> > >  #include "dcdbas.h"
> >
> > >                 /* Calling Interface SMI */
> > > -               smi_cmd->ebx = (u32) virt_to_phys(smi_cmd->command_buffer);
> > > +               smi_cmd->ebx = smi_data_buf_phys_addr
> > > +                               + offsetof(struct smi_cmd, command_buffer);
> >
> > Please, keep at least + on the previous line.
> > Also, I'm not sure what is the difference now. Especially for previous
> > users when this wasn't the part of the driver.
> > Some explanation needed.
> >
> > > +static u8 checksum(u8 *buffer, u8 length)
> > > +{
> > > +       u8 sum = 0;
> > > +       u8 *end = buffer + length;
> > > +
> > > +       while (buffer < end)
> > > +               sum = (u8)(sum + *(buffer++));
> >
> > Why not simple
> >
> > sum += *buffer++;
> >
> > ?
> >
> > > +       return sum;
> > > +}
> >
> > And I would rather check if we have similar algoritms already in the
> > kernel which  we might re-use.
> 
> Seems to be some options in lib/checksum.c to check.
> 
> >
> > > +
> > > +static inline struct smm_eps_table *check_eps_table(u8 *addr)
> > > +{
> > > +       struct smm_eps_table *eps = (struct smm_eps_table *)addr;
> > > +
> >
> > > +       if (strncmp(SMM_EPS_SIG, eps->smm_comm_buff_anchor, 4) != 0)
> >
> > I'm not sure about strings operation here.
> > I would rather do like with other magic constants: introduce hex value
> > and compare it as unsigned integer.
> >
> > Also, it might be a warning, since \0 wasn't ever checked from the
> > string literal. Though, I'm not sure if it applicable to strncmp()
> > function (it's for strncpy for sure).
> 
> I think we're OK here, and we're being consistent with the
> dell-wmi-descriptor test for "DELL WMI". I don't recall if it was that
> one or something else, but doing it in HEX ended up being more
> confusing. The \0 isn't an issue since strncmp will only compare the n
> (4) bytes.
> 
> >
> > > +               return NULL;
> > > +
> > > +       if (checksum(addr, eps->length) != 0)
> > > +               return NULL;
> > > +
> > > +       return eps;
> > > +}
> > > +
> > > +static int dcdbas_check_wsmt(void)
> > > +{
> > > +       struct acpi_table_wsmt *wsmt = NULL;
> > > +       struct smm_eps_table *eps = NULL;
> > > +       u8 *addr;
> > > +
> > > +       acpi_get_table(ACPI_SIG_WSMT, 0, (struct acpi_table_header **)&wsmt);
> > > +       if (!wsmt)
> > > +               return 0;
> > > +
> > > +       /* Check if WSMT ACPI table shows that protection is enabled */
> > > +       if (!(wsmt->protection_flags & ACPI_WSMT_FIXED_COMM_BUFFERS)
> > > +           || !(wsmt->protection_flags
> > > +                & ACPI_WSMT_COMM_BUFFER_NESTED_PTR_PROTECTION))
> > > +               return 0;
> > > +
> > > +       /* Scan for EPS (entry point structure) */
> > > +       for (addr = (u8 *)__va(0xf0000);
> > > +            addr < (u8 *)__va(0x100000 - sizeof(struct smm_eps_table)) && !eps;
> >
> > Perhaps better to do
> >
> > for (...) {
> >  eps = ...();
> >  if (eps)
> >   break;
> > }
> >
> > Also I've a feeling that 0xf0000 constant is defined already somewhere
> > under arch/x86/include/asm or evem include/linux.
> 
> But... is it defined for this purpose? If not, I'd prefer it hardcoded
> (or with a DEFINE).
> 
> >
> > > +            addr += 1)
> >
> > += 1?!
> > All tables I saw in BIOS are aligned with 16 bytes. Is it the case here?
> >
> > Is there any other means to check if the table present? ACPI code?
> > Method / variable?
> >
> > > +               eps = check_eps_table(addr);
> > > +
> > > +       if (!eps) {
> > > +               dev_dbg(&dcdbas_pdev->dev, "found WSMT, but no EPS found\n");
> > > +               return -ENODEV;
> > > +       }
> > > +
> > > +       /*
> > > +        * Get physical address of buffer and map to virtual address.
> > > +        * Table gives size in 4K pages, regardless of actual system page size.
> > > +        */
> >
> > > +       if (eps->smm_comm_buff_addr + 8 > U32_MAX) {
> >
> > if (upper_32_bits(..._addr + 8)) {
> >
> > ?
> >
> > > +               dev_warn(&dcdbas_pdev->dev, "found WSMT, but EPS buffer address
> is above 4GB\n");
> > > +               return -EINVAL;
> > > +       }
> > > +       eps_buffer = (u8 *)memremap(eps->smm_comm_buff_addr,
> >
> > Why casting?
> >
> > > +                                    eps->num_of_4k_pages * 4096, MEMREMAP_WB);
> >
> > This multiplication looks strange. Perhaps use PAGE_SIZE?
> >
> > > +       if (!eps_buffer) {
> > > +               dev_warn(&dcdbas_pdev->dev, "found WSMT, but failed to map EPS
> buffer\n");
> > > +               return -ENOMEM;
> > > +       }
> > > +
> > > +       /* First 8 bytes of buffer is for semaphore */
> > > +       smi_data_buf_phys_addr = (u32) eps->smm_comm_buff_addr + 8;
> >
> > lower_32_bits() ?
> >
> > > +       smi_data_buf = eps_buffer + 8;
> >
> > > +       smi_data_buf_size = (unsigned long) min(eps->num_of_4k_pages * 4096 -
> 8,
> > > +                           (u64) ULONG_MAX);
> >
> > This is too twisted code. First, it needs explanation.
> > Second, it might need some refactoring.
> >
> > (Yes, I got the idea, but would it be better implementation?)
> >
> > > +       max_smi_data_buf_size = smi_data_buf_size;
> > > +       wsmt_enabled = true;
> > > +       dev_info(&dcdbas_pdev->dev,
> > > +                "WSMT found, using firmware-provided SMI buffer.\n");
> > > +       return 1;
> > > +}
> >
> > >  #define SMI_CMD_MAGIC                          (0x534D4931)
> > >
> > > +#define SMM_EPS_SIG                            "$SCB"
> >
> > Just integer like above and put the sting as a comment.
> > (Side note: above magic also looks like string)
> 
> Given the above, I think we can use the more recognizable string - since
> that is clearly how they think of this label.
> 
> Otherwise, agree with a follow-up to all of Andy's feedback.
> 
> --
> Darren Hart
> VMware Open Source Technology Center

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