[PATCH v2 05/24] dt-bindings: net: dwmac: Elaborate stmmaceth/pclk description

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

Current clocks description doesn't provide a comprehensive notion about
what "stmmaceth" and "pclk" actually represent from the IP-core manual
point of view. The bindings file states:
stmmaceth - "GMAC main clock",
apb - "Peripheral registers interface clock".
It isn't that easy to understand what they actually mean especially seeing
the DW *MAC manual operates with clock definitions like Application,
System, Host, CSR, Transmit, Receive, etc clocks. Moreover the clocks
usage in the driver doesn't shade a full light on their essence. What
inferred from there is that the "stmmaceth" name has been assigned to the
common clock, which feeds both system and CSR interfaces. But what about
"apb"? The bindings defines it as the clock for "peripheral registers
interface". So it's close to the CSR clock in the IP-core manual notation.
If so then when "apb" clock is specified aside with the "stmmaceth", it
represents a case when the DW *MAC is synthesized with CSR_SLV_CLK=y
(separate system and CSR clocks). But even though the "apb" clock is
requested in the MAC driver, the driver doesn't actually use it as a
separate CSR clock where the IP-core manual requires. All of that makes me
thinking that the case of separate system and CSR clocks isn't correctly
implemented in the driver.

Let's start with elaborating the clocks description so anyone reading
the DW *MAC bindings file would understand that "stmmaceth" is the
system clock and "pclk" is actually the CSR clock. Indeed in accordance
with sheets depicted in [1]:
system/application clock can be either of: hclk_i, aclk_i, clk_app_i;
CSR clock can be either of: hclk_i, aclk_i, clk_app_i, clk_csr_i.
(Most likely the similar definitions present in the others IP-core
manuals.) So the CSR clock can be tied to the application clock
considering the later as the main clock, but not the other way around. In
case if there is only "stmmaceth" clock specified in a DT node, then it
will be considered as a source of clocks for both application and CSR. But
if "pclk" is also specified in the list of the device clocks, then it will
be perceived as the separate CSR clock.

[1] DesignWare Cores Ethernet MAC Universal Databook, Revision 3.73a,
    October 2013, p. 564.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
---
 .../devicetree/bindings/net/snps,dwmac.yaml          | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/net/snps,dwmac.yaml b/Documentation/devicetree/bindings/net/snps,dwmac.yaml
index 4dda9ffa822c..21e53427551c 100644
--- a/Documentation/devicetree/bindings/net/snps,dwmac.yaml
+++ b/Documentation/devicetree/bindings/net/snps,dwmac.yaml
@@ -116,8 +116,16 @@ properties:
     maxItems: 5
     additionalItems: true
     items:
-      - description: GMAC main clock
-      - description: Peripheral registers interface clock
+      - description:
+          GMAC main clock, also called as system/application clock.
+          This clock is used to provide a periodic signal for the DMA/MTL
+          interface and optionally for CSR, if the later isn't separately
+          clocked.
+      - description:
+          Peripheral registers interface clock, also called as CSR clock.
+          MCI, CSR and SMA interfaces run on this clock. If it's omitted,
+          the CSR interfaces are considered as synchronous to the system
+          clock domain.
       - description:
           PTP reference clock. This clock is used for programming the
           Timestamp Addend Register. If not passed then the system
-- 
2.29.2