RE: [PATCH 3/9] dma: qcom: bam_dma: Fix command element mask field for BAM v1.6.0+

[Lakshmi Sowjanya D (QUIC)]

> -----Original Message-----
> From: Md Sadre Alam <quic_mdalam@xxxxxxxxxxx>
> Sent: Friday, September 19, 2025 11:27 AM
> To: Konrad Dybcio <konrad.dybcio@xxxxxxxxxxxxxxxx>;
> broonie@xxxxxxxxxx; robh@xxxxxxxxxx; krzk+dt@xxxxxxxxxx;
> conor+dt@xxxxxxxxxx; andersson@xxxxxxxxxx; konradybcio@xxxxxxxxxx;
> vkoul@xxxxxxxxxx; linux-arm-msm@xxxxxxxxxxxxxxx; linux-spi@xxxxxxxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> dmaengine@xxxxxxxxxxxxxxx
> Cc: Varadarajan Narayanan (QUIC) <quic_varada@xxxxxxxxxxx>
> Subject: Re: [PATCH 3/9] dma: qcom: bam_dma: Fix command element mask
> field for BAM v1.6.0+
> 
> 
> 
> On 9/18/2025 3:57 PM, Konrad Dybcio wrote:
> > On 9/18/25 11:40 AM, Md Sadre Alam wrote:
> >> BAM version 1.6.0 and later changed the behavior of the mask field in
> >> command elements for read operations. In newer BAM versions, the mask
> >> field for read commands contains the upper 4 bits of the destination
> >> address to support 36-bit addressing, while for write commands it
> >> continues to function as a traditional write mask.
> >
> > So the hardware can read from higher addresses but not write to them?
> No,
> Write Operations: Can target any 32-bit address in the peripheral address
> space (up to 4GB)
> 
> Read Operations: Can read from any 32-bit peripheral address and place the
> data into 36-bit memory addresses (up to 64GB) starting from BAM v1.6.0
> >
> > Plus, you didn't explain what the mask register does on BAM <1.6.0.
> > If it really masks the address, all reads will now point to 0x0
> The mask field never masks addresses in any BAM version. Here's the
> complete specification:
> 
> BAM Command Element Structure
> 
> Write Command Elements (All BAM Versions):
> 
> | Field  | Bits  | Description                                         |
> |--------|-------|-----------------------------------------------------|
> | 1st DW | 31:24 | Command (must be 0 for write)                       |
> |        | 23:0  | Address - target address in peripheral              |
> |-----------------------------------------------------------------------
> | 2nd DW | 31:0  | Data - the data to be written                       |
> ------------------------------------------------------------------------
> | 3rd DW | 31:0  | Mask - 32-bit mask defining which bits to modify    |
> -----------------------------------------------------------------------
> | 4th DW | 31:0  | Reserved                                            |
> ------------------------------------------------------------------------
> 
> Read Command Elements (BAM < v1.6.0):
> 
> | Field  | Bits  | Description                                         |
> |--------|-------|-----------------------------------------------------|
> | 1st DW | 31:24 | Command (must be 1 for read)                        |
> |        | 23:0  | Address - source address in peripheral              |
> ------------------------------------------------------------------------
> | 2nd DW | 31:0  | Destination - memory address to write read-data     |
> ------------------------------------------------------------------------
> | 3rd DW | 31:0  | Reserved (IGNORED by hardware)                      |
> ------------------------------------------------------------------------
> | 4th DW | 31:0  | Reserved                                            |
> ------------------------------------------------------------------------
> 
> Read Command Elements (BAM >= v1.6.0):
> 
> | Field  | Bits  | Description                                         |
> |--------|-------|-----------------------------------------------------|
> | 1st DW | 31:24 | Command (must be 1 for read)                        |
> |        | 23:0  | Address - source address in peripheral              |
> ------------------------------------------------------------------------
> | 2nd DW | 31:0  | Destination - 32 LSBs of 36-bit destination addr    |
> ------------------------------------------------------------------------
> | 3rd DW | 31:4  | Reserved                                            |
> |        | 3:0   | Destination Address 4 MSBs (bits 35:32)             |
> ------------------------------------------------------------------------
> | 4th DW | 31:0  | Reserved                                            |
> ------------------------------------------------------------------------
> 
> For Read Commands:
> - BAM < v1.6.0: 3rd Dword completely ignored by hardware
> - BAM >= v1.6.0: 3rd Dword[3:0] contains upper 4 bits of destination address
> 
> Thanks,
> Alam.

Tested-by: Lakshmi Sowjanya D <quic_laksd@xxxxxxxxxxx>  # on SDX75

Thanks,
Lakshmi Sowjanya