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Date: Mon, 17 Jun 2024 20:57:10 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Len Brown <lenb@...nel.org>, Robert Moore <robert.moore@...el.com>,
linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, acpica-devel@...ts.linux.dev
Subject: Re: [PATCH 1/5] ACPI: sysfs: convert utf-16 from _STR to utf-8 only
once
On 2024-06-17 20:43:03+0000, Rafael J. Wysocki wrote:
> On Mon, Jun 17, 2024 at 8:37 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Thu, Jun 13, 2024 at 10:15 PM Thomas Weißschuh <linux@...ssschuh.net> wrote:
> > >
> > > The ACPI _STR method returns an UTF-16 string that is converted to utf-8
> > > before printing it in sysfs.
> > > Currently this conversion is performed every time the "description"
> > > sysfs attribute is read, which is not necessary.
> >
> > Why is it a problem, though?
It's not a real problem, mostly it made the following changes simpler.
> > How many devices have _STR and how much of the time is it used?
> >
> > Hint: On the system I'm sitting in front of, the answer is 0 and never.
>
> This was actually factually incorrect, sorry.
>
> The correct answer is 12 out of 247 and very rarely (if at all).
> Which doesn't really change the point IMO.
>
> > So Is it really worth adding an _STR string pointer to every struct acpi_device?
The string pointer replaces a 'union acpi_object *str_obj', so no new
space is used.
Also in case the device _STR is present the new code uses less memory, as
it doesn't need the full union and stores utf-8 instead of utf-16.
(Plus a few more cycles for the preemptive conversion)
In case no _STR is present both CPU and memory costs are identical.
Anyways, I don't really care about this and can also try to drop this
patch if you prefer.
Or drop the 'union acpi_device *' from the struct completely at your
preference.
> > > Only perform the conversion once and cache the result.
> > >
> > > Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> > > ---
> > > drivers/acpi/device_sysfs.c | 63 ++++++++++++++++++++++++++++-----------------
> > > include/acpi/acpi_bus.h | 2 +-
> > > 2 files changed, 40 insertions(+), 25 deletions(-)
> >
> > And it's more lines of code even.
> >
> > >
> > > diff --git a/drivers/acpi/device_sysfs.c b/drivers/acpi/device_sysfs.c
> > > index 23373faa35ec..4bedbe8f57ed 100644
> > > --- a/drivers/acpi/device_sysfs.c
> > > +++ b/drivers/acpi/device_sysfs.c
> > > @@ -439,24 +439,11 @@ static ssize_t description_show(struct device *dev,
> > > char *buf)
> > > {
> > > struct acpi_device *acpi_dev = to_acpi_device(dev);
> > > - int result;
> > >
> > > - if (acpi_dev->pnp.str_obj == NULL)
> > > + if (acpi_dev->pnp.str == NULL)
> > > return 0;
> > >
> > > - /*
> > > - * The _STR object contains a Unicode identifier for a device.
> > > - * We need to convert to utf-8 so it can be displayed.
> > > - */
> > > - result = utf16s_to_utf8s(
> > > - (wchar_t *)acpi_dev->pnp.str_obj->buffer.pointer,
> > > - acpi_dev->pnp.str_obj->buffer.length,
> > > - UTF16_LITTLE_ENDIAN, buf,
> > > - PAGE_SIZE - 1);
> > > -
> > > - buf[result++] = '\n';
> > > -
> > > - return result;
> > > + return sysfs_emit("%s\n", acpi_dev->pnp.str);
> > > }
> > > static DEVICE_ATTR_RO(description);
> > >
> > > @@ -507,14 +494,46 @@ static ssize_t status_show(struct device *dev, struct device_attribute *attr,
> > > }
> > > static DEVICE_ATTR_RO(status);
> > >
> > > +static const char *acpi_device_str(struct acpi_device *dev)
> > > +{
> > > + struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
> > > + union acpi_object *str_obj;
> > > + acpi_status status;
> > > + const char *ret;
> > > + char buf[512];
> > > + int result;
> > > +
> > > + if (!acpi_has_method(dev->handle, "_STR"))
> > > + return NULL;
> > > +
> > > + status = acpi_evaluate_object(dev->handle, "_STR",
> > > + NULL, &buffer);
> > > + if (ACPI_FAILURE(status))
> > > + return NULL;
> > > +
> > > + str_obj = buffer.pointer;
> > > + /*
> > > + * The _STR object contains a Unicode identifier for a device.
> > > + * We need to convert to utf-8 so it can be displayed.
> > > + */
> > > + result = utf16s_to_utf8s((wchar_t *)str_obj->buffer.pointer,
> > > + str_obj->buffer.length,
> > > + UTF16_LITTLE_ENDIAN,
> > > + buf, sizeof(buf) - 1);
> > > + buf[result++] = '\0';
> > > +
> > > + ret = kstrdup(buf, GFP_KERNEL);
> > > + kfree(buffer.pointer);
> > > +
> > > + return ret;
> > > +}
> > > +
> > > /**
> > > * acpi_device_setup_files - Create sysfs attributes of an ACPI device.
> > > * @dev: ACPI device object.
> > > */
> > > int acpi_device_setup_files(struct acpi_device *dev)
> > > {
> > > - struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
> > > - acpi_status status;
> > > int result = 0;
> > >
> > > /*
> > > @@ -539,12 +558,8 @@ int acpi_device_setup_files(struct acpi_device *dev)
> > > /*
> > > * If device has _STR, 'description' file is created
> > > */
> > > - if (acpi_has_method(dev->handle, "_STR")) {
> > > - status = acpi_evaluate_object(dev->handle, "_STR",
> > > - NULL, &buffer);
> > > - if (ACPI_FAILURE(status))
> > > - buffer.pointer = NULL;
> > > - dev->pnp.str_obj = buffer.pointer;
> > > + dev->pnp.str = acpi_device_str(dev);
> > > + if (dev->pnp.str) {
> > > result = device_create_file(&dev->dev, &dev_attr_description);
> > > if (result)
> > > goto end;
> > > @@ -618,7 +633,7 @@ void acpi_device_remove_files(struct acpi_device *dev)
> > > * If device has _STR, remove 'description' file
> > > */
> > > if (acpi_has_method(dev->handle, "_STR")) {
> > > - kfree(dev->pnp.str_obj);
> > > + kfree(dev->pnp.str);
> > > device_remove_file(&dev->dev, &dev_attr_description);
> > > }
> > > /*
> > > diff --git a/include/acpi/acpi_bus.h b/include/acpi/acpi_bus.h
> > > index 1a4dfd7a1c4a..32e3105c9ece 100644
> > > --- a/include/acpi/acpi_bus.h
> > > +++ b/include/acpi/acpi_bus.h
> > > @@ -254,7 +254,7 @@ struct acpi_device_pnp {
> > > struct list_head ids; /* _HID and _CIDs */
> > > acpi_device_name device_name; /* Driver-determined */
> > > acpi_device_class device_class; /* " */
> > > - union acpi_object *str_obj; /* unicode string for _STR method */
> > > + const char *str; /* _STR */
> > > };
> > >
> > > #define acpi_device_bid(d) ((d)->pnp.bus_id)
> > >
> > > --
> > > 2.45.2
> > >
> > >
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